pgsql: Improve sequence and sense of messages from pg_stop_backup().

Log Message:
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Improve sequence and sense of messages from pg_stop_backup().
Now doesn't report it is waiting until it actually is waiting,
plus message doesn't appear until at least 5 seconds wait, so
we avoid reporting the wait before we've given the archiver
a reasonable time to wake up and archive the file we just
created earlier in the function.
Also add new unconditional message to confirm safe completion.
Now a normal, healthy execution does not report waiting at
all, just safe completion.

Modified Files:
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    pgsql/src/backend/access/transam:
        xlog.c (r1.399 -> r1.400)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.399&r2=1.400)