Split the merger into a separate process

Connect it to Zuul via Gearman.  Any number of mergers may be
deployed.

Directly find the pipeline for a build when processing a result,
so that the procedure is roughly the same for build and merge
results.

The timer trigger currently requires the gerrit trigger also be
configured.  Make that explicit inside of the timer trigger so
that the scheduler API interaction with triggers is cleaner.

Change-Id: I69498813764753c97c426e42d17596c2ef1d87cf