deleteExpiredNtfs trimmed each notifier's message list but never removed
the outer NtfStore map key, so one empty entry per notifier queue that
ever received a notification was retained forever (grows with the active
notifier set, never shrinks).
Remove the outer key when its list becomes empty, and make storeNtf fully
atomic so it cannot race the removal and write a notification to an
orphaned TVar. Verified with the load bench (ntfexp): after expiry
ntfStore_keys drops from the queue count to 0 instead of staying flat.
* .401
* stats for undelivered notifications
* logs, stats
* control port show ntf client IDs
* check that Ntf client is still current and that queue is not full, drop notifications otherwise
* prevent losing notifications when client is not current or queue full
* add log when no notifications, remove some logs
* reduce STM transaction
* revert version change
shum