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
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 4b7b4b0..c5beda0 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -20,6 +20,7 @@
 
    gating
    triggers
+   merger
    launchers
    reporters
    zuul