Slow memory leak in dealer.py

[DZabavchik]

Finally found this leak that was haunting for 7+ years

crossbar/crossbar/router/dealer.py

Lines 1135 to 1139 in 0089c1e

	# the session_id will be None if the caller session has
	# already vanished
	caller_id = invocation_request.caller._session_id
	if caller_id is not None:
	del self._invocations_by_call[caller_id, invocation_request.call.request]

Zombie invocations remain in _invocations_by_call with references to callee, caller, authorization that will not be GC'd

Testing changes that preserve original caller session ID in InvocationRequest object, so it can be deleted by key from _invocations_by_call with
del self._invocations_by_call[invocation_request.caller_session_id, invocation_request.call.request]

Will submit PR shortly