Fix checkpoint signalling

Fix checkpoint signalling

Checkpointer uses its MyLatch to wake up when a checkpoint request is
received.  But before commit c6550776394e the latch was not used for
anything else, so the code could just go to sleep after each loop
without rechecking the sleeping condition.  That commit added a separate
ResetLatch in its code path[1], which can cause a checkpoint to go
unnoticed for potentially a long time.

Fix by skipping sleep if any checkpoint flags are set.  Also add a test
to verify this; authored by Kyotaro Horiguchi.

[1] CreateCheckPoint -> InvalidateObsoleteReplicationSlots ->
ConditionVariableTimeSleep

Report and diagnosis by Kyotaro Horiguchi.
Co-authored-by: Kyotaro Horiguchi <[email protected]>
Co-authored-by: Álvaro Herrera <[email protected]>
Discussion: https://postgr.es/m/20200408.141956.891237856186513376[email protected]