Autovacuum worker pool saturation delays emergency vacuum completion

Emergency vacuums triggered by MultiXact member space exhaustion can exceed available autovacuum worker threads. When multiple large tables are enqueued for aggressive vacuum simultaneously, worker pool saturation causes sequential processing instead of parallel, extending recovery time from hours to many hours.

Increase number of autovacuum worker threads to handle concurrent emergency vacuums. Add monitoring and alerting for autovacuum worker pool saturation. Monitor autovacuum_timestamp to track vacuum queue depth. During high-write operations, proactively ensure worker capacity is available.