pgsql: In walsender, don't sleep if there's outstanding WAL waiting to

Log Message:
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In walsender, don't sleep if there's outstanding WAL waiting to be sent,
otherwise we effectively rate-limit the streaming as pointed out by
Simon Riggs. Also, send the WAL in smaller chunks, to respond to signals
more promptly.

Modified Files:
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    pgsql/src/backend/replication:
        walsender.c (r1.20 -> r1.21)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walsender.c?r1=1.20&r2=1.21)