[Bug] Ensure that callbacks are never called when subscriber/queryable/session is dropped #986
Description
Describe the bug
Ensure that callbacks are never called when subscriber/queryable/session is dropped. Also ensure that these callbacks will be dropped before corresponding drop/undeclare call returns.
Calling callbacks after subscriber/queryable destruction can result in UB, since there is no borrow checker to ensure that data captured by callbacks is still alive at the moment of callback invocation/destruction.
To reproduce
It is sufficient to undeclare subscriber/queryable roughly at the same moment when a new message/query arrives.
System info
any