perf: release RLock before blocking select in deliver

[hawkadrian]

What

Release RLock() before the blocking select in deliver(). Copy postC to a local variable under the lock, then release it before channel operations. Add recover() to handle the race where the channel may be closed between copying and using it.

Why

Prevents closewait() from blocking when waiting for Lock() while deliver() holds RLock() during channel operations. This improves responsiveness when closing subscriptions under high concurrency.