[core worker] Python core worker task execution (#5783)

Executes tasks via the event loop in the C++ core worker. Also properly handles signals (including KeyboardInterrupt), so ctrl-C in a python interactive shell works now (if connecting to an existing cluster).

python/ray/experimental/async_plasma.py

@@ -199,8 +199,8 @@ class PlasmaEventHandler:
         del self._waiting_dict[fut.object_id]
 
     def _complete_future(self, fut):
-        obj = self._worker.retrieve_and_deserialize(
-            [ray.ObjectID(fut.object_id.binary())], 0)[0]
+        obj = self._worker.get_objects([ray.ObjectID(
+            fut.object_id.binary())])[0]
         fut.set_result(obj)
 
     def as_future(self, object_id, check_ready=True):