Add wait events for checkpoint delay mechanism.

Add wait events for checkpoint delay mechanism.

When MyProc->delayChkptFlags is set to temporarily block phase
transitions in a concurrent checkpoint, the checkpointer enters a
sleep-poll loop to wait for the flag to be cleared.  We should show that
as a wait event in the pg_stat_activity view.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CA%2BhUKGL7Whi8iwKbzkbn_1fixH3Yy8aAPz7mfq6Hpj7FeJrKMg%40mail.gmail.com