Don't auto-restart per-database autoprewarm workers.

We should try to prewarm each database only once.  Otherwise, if
prewarming fails for some reason, it will just keep retrying in an
infnite loop.  This can happen if, for example, the database has been
dropped.  The existing code was intended to implement the try-once
behavior, but failed to do so because it neglected to set
worker.bgw_restart_time to BGW_NEVER_RESTART.

Mithun Cy, per a report from Hans Buschmann

Discussion: http://postgr.es/m/CA+hUKGKpQJCWcgyy3QTC9vdn6uKAR_8r__A-MMm2GYfj45caag@mail.gmail.com

contrib/pg_prewarm/autoprewarm.c

@@ -840,6 +840,7 @@ apw_start_database_worker(void)
 	worker.bgw_flags =
 		BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
 	worker.bgw_start_time = BgWorkerStart_ConsistentState;
+	worker.bgw_restart_time = BGW_NEVER_RESTART;
 	strcpy(worker.bgw_library_name, "pg_prewarm");
 	strcpy(worker.bgw_function_name, "autoprewarm_database_main");
 	strcpy(worker.bgw_name, "autoprewarm worker");