pgsql: Make autovacuum report the start time of its current activity in

Log Message:
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Make autovacuum report the start time of its current activity in
pg_stat_activity. Per gripe from Jim Nasby.

Modified Files:
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pgsql/src/backend/postmaster:
autovacuum.c (r1.58 -> r1.59)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.58&r2=1.59)