A bigger revision than I'd planned.

* extract eventdb from watchdb and setup a connection pool for redis
* preliminary work on matching DSNs (required for side-by-side servers)
* add more detail to request tracing (with support for adding more)
* add --verbose option to cut down on noise
* add automatic tag generation (fixes #4)
* simplified the task dispatcher in goose/worker
* removed a bunch of worker interfaces that were just annoying to deal with
* add DELETE /watches endpoint to flush the watchdb

Some of the code was getting twisty and brittle, hence the largish number of changes.