It can happen that ::get_request() returns NULL if the
EventPool is full. In that case the slot is never called.
In this case the caller can now take action.
Some overzealous locking to track down RequestObject related crashes.
bc0fa4d689 wrongly locked the current event loop's
request_invalidation_lock instead of the invalidation's list lock.
Also Abstract UI is able to delete requests concurrently with with
EventLoop invalidation.
e.g. PortManager::PortRegisteredOrUnregistered and GlobalPortMatrixWindow
so the lock needs to be exposed.
If this solves various issues, mutexes should to be consolidated
(request_buffer_map_lock + request_invalidation_lock) and be chosen
such that there is as little contention as possible.
