Clean up mailbox map in mailbox service if a query has finished

[walterddr]

• mailbox reference will be kept in concurrent hashmap forever
• if query fails, it doesn't cancel all the mailboxes, but the mailbox receive operator already returned. this means any incoming messages for that specific mailbox receive operator will be buffered in the queue forever.
• this cannot be a shutdown time clean up as the long running service will keep piling up new mailboxes as requestID grows (they are not reusable)