Report pipeline stats.

 * Number of changes processed.
 * Time each change spent in pipeline.
 * Current number of changes in pipeline.

All of those per-pipeline, and per-pipeline, per-project.

Change-Id: I609ff33ab7c26fc43d323f8a0bbb61f93382af90
Reviewed-on: https://review.openstack.org/19764
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins