Teach RequestCheckpoint() to wait and retry a few times if it can't signal

Teach RequestCheckpoint() to wait and retry a few times if it can't signal
the bgwriter immediately.  This covers the case where the bgwriter is still
starting up, as seen in a recent buildfarm failure.  In future it might also
assist with clean recovery after a bgwriter termination and restart ---
right now the postmaster treats early bgwriter exit as a system crash,
but that might not always be so.