Fixed #10088 -- Added QuerySet.select_for_share().

[ngnpope]

Initial work to address ticket-10088.

Alternative to #15939 and #16172.

Thought I'd get this out there as I've been sitting on it for a couple of years. I've rebased it.

• Adds support for nowait, skip_locked, of and key options
  • Note that select_for_update() has no_key, but select_for_share() has key
• Adds support for LOCK IN SHARE MODE for MariaDB ~and MySQL < 8.0.1~ (Django 5.0 will support MySQL 8.0.11+)
• Implements support of for_share_after_from like for_update_after_from used by MSSQL backend
• Shared documentation for select_for_share() and select_for_update() as very similar
• Shared tests for select_for_share() and select_for_update() as very similar

The patch is quite mature, but there is still some work to be done:

• [ ] Properly implement the "blocking" tests using other thread for FOR SHARE case
• [x] Determine what to do about subTest() not supporting skipTest()
  • Have split into separate test modules with some preliminary refactoring to make things more maintainable.
• [x] Review whether anything has been missed. I've rebased this but it was written a long time ago...

I've marked this as a draft because of needing to think about the testing issues, but it's ready for a first round of review.

[ngnpope]

Have pushed progress so far. Still some tidying up to be done, fixing some tests, etc.

So no review required yet.

[ngnpope]

buildbot, test on oracle.