PostgreSQL 19 adds pg_stat_lock for lock contention monitoring

PostgreSQL 19 introduces the pg_stat_lock view with cumulative lock statistics: wait counts, total wait time and fast-path slot overflow. Only waits longer than deadlock_timeout that end in a successful lock acquisition are counted.

• pg_stat_lock returns 12 rows, one per lock type
• Only waits longer than deadlock_timeout ending in success are counted
• fastpath_exceeded tracks exhaustion of fast-path slots
• Statistics reset via pg_stat_reset_shared('lock')