By providing some custom handling for ProcArrayEndTransaction, we can avoid the need for ending transactions to repeatedly acquire the spinlock. The amount of work that needs to be done while holding the lock is so small that we can do it while holding the spinlock, or (when the lock is contended) make the last person to release the lock do it on behalf of the ending backend. This greatly improves performance for unlogged tables at high client counts; permanent tables also benefit, but performance is still severely throttled by WALInsertLock contention.