pgsql: Teach RequestCheckpoint() to wait and retry a few times if it

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.53 -> r1.54)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c?r1=1.53&r2=1.54)