pgsql: Make NOTIFY_PAYLOAD_MAX_LENGTH depend explicitly on BLCKSZ and

Log Message:
-----------
Make NOTIFY_PAYLOAD_MAX_LENGTH depend explicitly on BLCKSZ and
NAMEDATALEN, so this code doesn't go nuts with smaller than default
BLCKSZ or larger than default NAMEDATALEN.  The standard value is
still exactly 8000.

Modified Files:
--------------
    pgsql/src/backend/commands:
        async.c (r1.151 -> r1.152)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/async.c?r1=1.151&r2=1.152)