pgsql: Use WaitLatch() instead of pg_usleep() at end-of-vacuum truncati

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/70685385d70f8da73ab189a72f46311091ff09be

Modified Files
--------------
doc/src/sgml/monitoring.sgml            | 5 +++++
src/backend/access/heap/vacuumlazy.c    | 6 +++++-
src/backend/utils/activity/wait_event.c | 3 +++
src/include/utils/wait_event.h          | 3 ++-
4 files changed, 15 insertions(+), 2 deletions(-)