pgsql: Fix a problem in my recent patch to initialize cancel_key for

Log Message:
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Fix a problem in my recent patch to initialize cancel_key for autovac workers
as well as regular backends: if no regular backend launches before the autovac
launcher tries to start an autovac worker, the postmaster would get an Assert
fault due to calling PostmasterRandom before random_seed was initialized.
Cleanest solution seems to be to take the initialization of random_seed out
of ServerLoop and let PostmasterRandom do it for itself.

Modified Files:
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    pgsql/src/backend/postmaster:
        postmaster.c (r1.538 -> r1.539)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.538&r2=1.539)