Bug heat calculation should be done using a stored procedure

What happens:
If calculate-bug-heat.py doesn't run for 24 hours or so (say during rollouts and re-rolls) then a lot of bugs' heat gets outdated. Once the garbo script has run and dumped a bunch of CalculateBugHeatJobs into the queue, c-b-h can't cope with the volume and appears to get stuck.

What should happen:
The problem is that we try to calculate bug heat serially, even for outdated bugs (of which there are at least hundreds even on a quiet day). So, c-b-h.py tries to go through the several thousand jobs in its queue one by one and never gets finished. There are ways to mitigate this problem but it would be simpler to do the calculations all at once as part of garbo-hourly or -daily.

We should replace the jobs-based bug heat calculation code with a stored procedure. This could work for both re-calculations of outdated bug heat (by being called for all out of date bugs by garbo) and on-change recalculations.