Schedule PackageCopyJob to run more frequently.

[Gavin Panella]

PackageCopyJob is in the MAIN job source group, which is scheduled to run every 5 minutes on loganberry. We may want package copies to happen more frequently, so we could introduce a FREQUENT (for example) job source group and move IPackageCopyJobSource into it. The LOSAs could then schedule this to run every minute or two.

[Julian Edwards (julian-edwards)]

Just a thought, but it'd be nice to have a twistd job runner that users PSQL
triggers, so there's no delay at all.

[Gavin Panella (allenap)]

I suggested using LISTEN/NOTIFY <http://www.postgresql.org/docs/8.4/interactive/sql-notify.html> at one time, but ISTR that stub had some valid arguments why we shouldn't.

[Gavin Panella (allenap)]

To ensure that package copies continue without interruption, and
without need for any time-dependent changes, like changing a crontab
at the same time that a Launchpad change is deployed, the plan is to
land four branches as follows:

1. To Launchpad: Make --exclude work with process-job-source-groups.py.

2. To Launchpad's crontabs: Change IPlainPackageCopyJobSource to run
   on ackee every minute and stop running it on loganberry. The latter
   will be done with --exclude.

3. To Launchpad: Change IPlainPackageCopyJobSource to be in the
   FREQUENT job source group.

4. To Launchpad's crontabs: Remove the --exclude from the crontab on
   loganberry.

Each branch needs to land and be deployed before the next can be
rolled through.