multiprocessing's Condition uses a couple of semaphores to keep track of
processes which are sleeping and have woken up whilst waiting for the
condition. These counters however are only ever decremented in the
notify(_all) functions, via a loop which results in somewhat unexpected
behaviour where said functions take longer to run than expected.

The proposed patch simplifies notify(_all) functions such that time complexity
is still O(N) but crucially N is the number of currently sleeping processes
only (rather than number of wait() calls since last notify(_all) call).

Note: This also affects Event.wait() & Event.set() in the same fashion since a
Condition is used under the hood.

I've attached mp_sync_condition.patch based on "in-development" branch as of
98840:2028aed246c0. I have run unit tests on said commit (via debug build)
using:

./python -bb -E -Wd -m test -r -v -uall\
    test_multiprocessing_fork\
    test_multiprocessing_fork\
    server test_multiprocessing_spawn

Additionally I've run condition_test.py before & after to illustrate performance
of the proposed change as well as demonstrate the issue.