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