Wait events for delayed checkpoints

Hi,

You can't tell if your checkpointer is spending a lot of time waiting
around for flags in delayChkptFlags to clear. Trivial patch to add
that. I've managed to see it a few times when checkpointing
repeatedly with a heavy pgbench workload.

I had to stop and think for a moment about whether these events belong
under "WaitEventIPC", "waiting for notification from another process"
or under "WaitEventTimeout", "waiting for a timeout to expire". I
mean, both? It's using sleep-and-poll instead of (say) a CV due to
the economics, we want to make the other side as cheap as possible, so
we don't care about making the checkpointer take some micro-naps in
this case. I feel like the key point here is that it's waiting for
another process to do stuff and unblock it.