pgsql: Use WaitLatch() for condition variables.

Use WaitLatch() for condition variables.

Previously, condition_variable.c created a long lived WaitEventSet to
avoid extra system calls.  WaitLatch() now uses something similar
internally, so there is no point in wasting an extra kernel descriptor.

Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e2d394df5df28ab5ee4bfac6b13837e99e24045d

Modified Files
--------------
src/backend/storage/lmgr/condition_variable.c | 28 +++++----------------------
1 file changed, 5 insertions(+), 23 deletions(-)