Fix several Postgresql errors

[hannahhoward]

Goals

Solve running dealbot issues that reference pq -- i.e. our postgresql library

Implementation

• setup a test harness that allows us to run our state tests with either SQLite or PostgreSQL. The default remains SQLite, but if the tests are run with POSTGRES_BIN env var set (a bin dir for PG's various executables), then each test will make a temporary test PostgresSQL cluster and execute the tests with a PG connector. this allows us to test locally at least for PG when adding state features
• Also add a state task for draining + completing workers
• Now solve the various issues with PG:
  • Concurrent assign task calls were not working cause we weren't using serializable transactions
  • But at the same time, our PG RetryableError method had an issue that needed fixing
  • Fix the query with oldestAvailableTaskWithTagsSQL for PG that is causing 500's on popTask on the dealbot
  • Fix ResetWorkerTasks and Complete for PSQL which has to do with the fact that row queries must be complete before doing further operations while in a transaction (this is potentially somewhat memory inefficient -- we can certainly improve it with PSQL cursors if it becomes an issue, but for now, probably best to just do the simple thing)
    Add test harness for PSQL and verify all database operations work with PSQL