Properly manage the lifetime of Exiters in Daemon Runs

[blorente]

Problem

There is a bug where, once the LocalPantsRunner has reset the global exiter to its own LocalExiter, it won't un-set it ever, and therefore unhandled exceptions will use that exiter to exit.

This has caused issues, around this line, because the LocalPantsRunner reset the global exiter and the DaemonPantsRunner didn't reset it.

There is a regression test to demonstrate this issue.

Solution

• Following the discussion on wip - add contextmanager'd exiters and change return type of run() #7606, implement a contextmanager that temporarily overrides the global exiter.
• Remove the concept of "my exiter", and make classes (LocalPantsRunner, DaemonPantsRunner, PantsRunner and PantsDaemon) use the global exiter.

Result

The regression test passes, and _log_unhandled_exceptions_and_exit works as expected in DaemonPantsRunner.

[ity]

I understand the time constraint with this review, so if you want to create a ticket and circle back with the cosmetic changes suggested, that would be completely fine!

[blorente]

I have made many changes to the PR, so I'd like a fresh review if possible.
Every commit is independently reviewable.

[stuhood]

Thanks a lot! This looks good.

[blorente]

Will merge on green.