Add logging to the zuul merger process that includes change numbers so
that threads of execution can be easily tracked from the zuul server log
files to the zuul merger log files.
Connect it to Zuul via Gearman. Any number of mergers may be
Directly find the pipeline for a build when processing a result,
so that the procedure is roughly the same for build and merge
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.