When possible, avoid using context keys skipping the whole mail.thread
stack. Since a few years various usage of those keys have been added
through odoo addons quite often without clear purpose in mind when
asked to authors or when reading commit messages. Partly because
people tend to copy-paste code patterns without really understanding
the purpose of those.

See individual commits for more details.

USAGE IN TESTS

Lots of tests use the 'tracking_disable' or 'mail_notrack' context
keys. However I bet most of those are there just because they
were copy pasted, and without any thinking about the usage

 * it is used on non-thread models (which shows writer did not check
   what it was about);
 * it is copy-pasted in multiple unit tests creating one data each
   time (which shows performance are not the matter here as the
   writer could use a setupClass);
 * most usage is done when creating records, although there is no
   tracking at create time. And even if someday tracking at create
   comes back it would not be a performance issue on a test db
   compared to current test workload (too much tests, tours that
   are slow, ...). It also deactivates creation message log and
   initial follower (if not root) but those insert should be fast;
 * mail.thread is part of the real life stack and should be tested
   in functional addons. It notably has an impact on followers
   which means ACLs, partner_id field setup, field computation
   and invalidation, cache usage, ...

Better remove most of them, and keep only relevant one (e.g. batch
creation, simulating environment like Payroll, ...).

Task-6094598
Followup of Task-3645865