We had a bug where if we had forked (in the ProcessLauncherPosixFork)
while another thread was writing a log message, we would deadlock. This
happened because the fork child inherited the locked log rwmutex, which
would never get unlocked. This meant the child got stuck trying to
disable all log channels.
The bug existed for a while but only started being apparent after
D37930, which started using ThreadLauncher (which uses logging) instead
of std::thread (which does not) for launching TaskPool threads.
The fix is to provide a lock free way to disable logging, which should
be used in this case, and this case only.