[13.0] Optimize queue.job creation

[guewen]

Several fields on queue.job are initialized using computed fields,
then never changed again. On creation of a queue.job record, we'll have
an initial INSERT + at least one following UPDATE for the computed
fields.

Replace all the stored computed fields by a raw initialization of the
values in Job.store() when the job is created, so we have only a single
INSERT.

Highlights:

• as channel is no longer a compute/inverse field, override_channel is
  useless, I dropped it (actually the same value was stored in both
  channel and override_channel as the channel field was stored)
• one functional diff is that now, when changing a channel on a
  job.function, the channel is no longer synchronized on existing jobs,
  it will be applied only on new jobs: actually this is an improvement,
  because it was impossible to change the channel of a job function
  in a large queue_job table as it meant writing on all the done/started
  jobs
• searching the queue.job.function is now cached, as each job using the
  same will run a query on queue_job_function

Rough benchmark when generating jobs for ddmrp buffers (it includes some base
time to fetch the ddmrp buffers too). 9191 jobs were created:

executed with	duration
no optimizations	2m18s
create optimization (current PR)	1m41s
mail.message/follower optimization (#302)	1m11s
both optimizations	35s

[guewen]

Added a small benchmark in the description

[simahawk]

/ocabot merge minor

[OCA-git-bot]

This PR looks fantastic, let's merge it!
Prepared branch 13.0-ocabot-merge-pr-305-by-simahawk-bump-minor, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at 15dc56f. Thanks a lot for contributing to OCA. ❤️