Worker: Method for selecting task to work on

[dwhswenson]

The Worker need to have a way to select which task it will run. There are a few options here; I think we should engineer things such that we can easily try alternatives, since I'm not sure what will best meet needs of users. A couple options:

• Priority in the task status DB. Get first available sorted by priority (on the SQL end). Should be fast, but reading doesn't block other readers, so there are potential concurrency pileup issues.
• Make the decision in Python; could include some randomness to avoid concurrency issues (e.g., select weighted by priority). Will be slower between read and claim, but we already have safety on the claim to ensure that we're actually the only one to get stake our claim to a task.