Use WaitLatch() instead of pg_usleep() at end-of-vacuum truncation

Use WaitLatch() instead of pg_usleep() at end-of-vacuum truncation

This has the advantage to make a process more responsive when the
postmaster dies, even if the wait time was rather limited as there was
only a 50ms timeout here.  Another advantage of this change is for
monitoring, as we gain a new wait event for the end-of-vacuum
truncation.

Author: Bharath Rupireddy
Reviewed-by: Aleksander Alekseev, Thomas Munro, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACU4AdPCq6NLfcA-ZGwX7pPCK5FgEj-CAU0xCKzkASSy_A@mail.gmail.com