gerrit.Watcher is a class which watches the Gerrit stream-events SSH
connection and produces events.
There's a basic CLBot binary as well, to demonstrate driving it to
produce messages on the logging output. It doesn't really do anything
else.
Change-Id: I274fe0a77c8329f79456425405e2fbdc3ca2edf0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/245
Reviewed-by: tazjin <mail@tazj.in>