Clean up when a driver disconnects. (#462)

* Clean up state when drivers exit.

* Remove unnecessary field in ActorMapEntry struct.

* Have monitor release GPU resources in Redis when driver exits.

* Enable multiple drivers in multi-node tests and test driver cleanup.

* Make redis GPU allocation a redis transaction and small cleanups.

* Fix multi-node test.

* Small cleanups.

* Make global scheduler take node_ip_address so it appears in the right place in the client table.

* Cleanups.

* Fix linting and cleanups in local scheduler.

* Fix removed_driver_test.

* Fix bug related to vector -> list.

* Fix linting.

* Cleanup.

* Fix multi node tests.

* Fix jenkins tests.

* Add another multi node test with many drivers.

* Fix linting.

* Make the actor creation notification a flatbuffer message.

* Revert "Make the actor creation notification a flatbuffer message."

This reverts commit af99099c8084dbf9177fb4e34c0c9b1a12c78f39.

* Add comment explaining flatbuffer problems.

src/common/common.h

@@ -153,7 +153,9 @@ extern const UniqueID NIL_ID;
 UniqueID globally_unique_id(void);
 
 #define NIL_OBJECT_ID NIL_ID
+#define NIL_WORKER_ID NIL_ID
 
+/** The object ID is the type used to identify objects. */
 typedef UniqueID ObjectID;
 
 #ifdef __cplusplus
@@ -202,6 +204,18 @@ bool ObjectID_equal(ObjectID first_id, ObjectID second_id);
  */
 bool ObjectID_is_nil(ObjectID id);
 
+/** The worker ID is the ID of a worker or driver. */
+typedef UniqueID WorkerID;
+
+/**
+ * Compare two worker IDs.
+ *
+ * @param first_id The first worker ID to compare.
+ * @param second_id The first worker ID to compare.
+ * @return True if the worker IDs are the same and false otherwise.
+ */
+bool WorkerID_equal(WorkerID first_id, WorkerID second_id);
+
 typedef UniqueID DBClientID;
 
 /**