[Cleanup] Remove insertion-lock RPCs and locked-retry transport

[thep2p]

Summary

Skip Graph joins no longer take pessimistic locks. Under Aspnes & Shah, "Skip Graphs" (arXiv:cs/0306043), Algorithm 2 joins a node by splicing it into each level's doubly-linked list through order-verification message forwarding — no distributed locking, no lock ordering, no random backoff. With the lock-free join in place, the request/response machinery that coordinated the old pessimistic lock is dead weight.

This issue removes the lock-coordination RPCs (AcquireLock, ReleaseLock, IsAvailable) and the "locked -> back off and retry" transport path in the middle layer. Removing the retry loop also eliminates the code path that dereferences a possibly-null response, which is the subject of #30.

This is the first of two cleanup steps and strips every remote consumer of the node-level lock methods. After this change, SkipNode's tryAcquire / unlock / isLocked / isLockedBy overrides lose their remote callers but remain in place (they still delegate to InsertionLock); the InsertionLock class itself stays wired into SkipNode — its field, the constructor's startInsertion(), insert()'s endInsertion(), and isAvailable() — and is removed in the follow-up. Consumers must go before the class: deleting InsertionLock while these callers still reference it would not compile.

Scope

• src/main/java/underlay/packets/RequestType.java — remove the AcquireLock, ReleaseLock, and IsAvailable enum constants. Leave the unrelated AcquireNeighbors constant untouched.
• src/main/java/underlay/packets/requests/AcquireLockRequest.java, ReleaseLockRequest.java, IsAvailableRequest.java — delete.
• src/main/java/middlelayer/MiddleLayer.java:
  • receive(): remove the AcquireLock, ReleaseLock, and IsAvailable cases and the now-unused AcquireLockRequest / ReleaseLockRequest / IsAvailableRequest imports.
  • Remove the client senders tryAcquire(...) and both unlock(...) overloads (their only callers live in the lock-based join path removed by the join rewrite), and both isAvailable(...) overloads (already dead — they have no caller anywhere in the current code).
  • send(): replace the do { ... } while (request.backoff && response.locked) backoff-retry loop (with its random Thread.sleep) with a single delivery of the request.
  • Remove the per-case routing gates: the !overlay.isAvailable() gate on the SearchByMembershipVector / SearchByMembershipVectorRecursive / SearchByIdentifier cases, and the overlay.isLocked() && !overlay.isLockedBy(...) gate on the UpdateLeftNode / UpdateRightNode / GetRightNode / GetLeftNode / FindLadder cases (each case now runs unconditionally and no longer returns a locked response).
  • Collapse each neighbor getter to a single method. getRightNeighborOf currently has three overloads — a dead getRightNeighborOf(String, int, int) with no caller, a getRightNeighborOf(String, int, Identifier, int) delegator that passes backoff = true, and the getRightNeighborOf(boolean, String, int, Identifier, int) implementation. Delete the dead 3-argument overload and merge the delegator and the implementation into a single getRightNeighborOf(String, int, Identifier, int) that sends the request once and returns the identity, dropping the boolean backoff parameter, the req.backoff = backoff plumbing, and the if (r.locked) return INVALID_NODE branch. Apply the same collapse to getLeftNeighborOf: merge its true-passing getLeftNeighborOf(String, int, Identifier, int) delegator and its getLeftNeighborOf(boolean, ...) implementation into one getLeftNeighborOf(String, int, Identifier, int) (there is no dead 3-argument left overload to remove). Dropping the boolean parameter without merging would leave two methods with identical signatures, so the merge must happen in the same change.
• src/main/java/skipnode/SkipNode.java — in findLadder(...), update the getLeftNeighborOf / getRightNeighborOf call sites to the merged 4-argument form (drop the leading false argument) and remove the now-dead INVALID_NODE (locked) short-circuit that followed them.

Note: the Request.backoff and Response.locked fields are intentionally left in place in this issue — once nothing reads them they compile cleanly as unused fields — and are deleted in the follow-up, to keep this change reviewable and within its size budget.

Acceptance Criteria

• AcquireLock, ReleaseLock, and IsAvailable no longer appear in RequestType, and the three request classes are deleted.
• MiddleLayer.receive() has no AcquireLock / ReleaseLock / IsAvailable cases and no isAvailable / isLocked / isLockedBy routing gates; every remaining case performs its operation unconditionally.
• MiddleLayer.send() delivers each request exactly once (no retry loop, no random sleep) and no longer dereferences a possibly-null response to decide on a locked-retry.
• getLeftNeighborOf and getRightNeighborOf are each a single method that no longer branches on a locked response; the dead getRightNeighborOf(String, int, int) overload is gone; no code reads request.backoff or response.locked (the fields themselves are removed in the follow-up).
• A repo-wide search finds no remaining references to AcquireLockRequest, ReleaseLockRequest, or IsAvailableRequest.
• The project compiles and the existing test suite passes with no new failures (SkipNodeTest, MvpTest, ConcurrentLookupTableTest).
• A new deterministic test (no sleeps) asserts that a request routed through the middle layer is delivered and answered in a single round-trip, with no locked/retry response involved.

Dependencies

• Requires the lock-free join rewrite to have landed first, so the insertion path no longer acquires or releases neighbor locks; otherwise the removed tryAcquire / unlock senders are still called.
• Related: [Bug] Cannot read field "locked" because "response" is null #30 — removing the backoff-retry loop eliminates the null-response dereference reported there.
• Must land before the follow-up cleanup that deletes the InsertionLock class and the node-level lock methods (those overrides become caller-less here but are only removed there).

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Part of #33. Depends on: #39