use weak ref for event loop to avoid leaking it

Summary: This dict holds strong reference to all the loops and leads to leak if new loop is created and used, change it to weakref to avoid it.

Reviewed By: yfeldblum

Differential Revision: D6892775

fbshipit-source-id: a628a3743876ef0580acce5fb9f283da573cf459

folly/python/executor.pyx

@@ -3,9 +3,10 @@ from folly cimport cFollyExecutor
 from folly.executor cimport cNotificationQueueExecutor
 from libcpp.memory cimport make_unique, unique_ptr
 from cython.operator cimport dereference as deref
+from weakref import WeakKeyDictionary
 
 #asynico Loops to NotificationQueueExecutor
-loop_to_q = {}
+loop_to_q = WeakKeyDictionary()
 
 
 cdef class NotificationQueueExecutor: