Tap: testing (#1333)

Test suite for Tap. This isn't fully-featured. Specifically, it doesn't
do full integration testing for event/repo processing. So doesn't test
most sync 1.1 semantics.

However it does test:
- outbox, delivery, and event ordering
- the basics of the firehose state machine
- the resync system
  - claiming resync jobs
  - draining the resync buffer

Snagged a couple of bug fixes along the way as well.

Author: dholms