[autotest] Time each step of scheduler tick and some database queries

We are interested in seeing how much impact migrating to CloudSQL
will have on the performance of the scheduler.
In this CL, we are adding some timing stats to the following places:
- each sub-step in tick() of the scheduler. The stats actually
  sit in site_* files.
- some database queries in host_scheduler.
- where job/host statuses get updated in scheduler_models.

BUG=chromium:196392
DEPLOY=scheduler
TEST=scheduler works and stat results are shown in graphite.

Change-Id: If3a142a0c1135ead6b36b35f2781037be16a90aa
Reviewed-on: https://gerrit.chromium.org/gerrit/47375
Tested-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Fang Deng <fdeng@chromium.org>
5 files changed