Switch to updated Plasma API and consolidate wait and fetch implementations. (#116)

* Consolidate wait implementations.

* Consolidate fetch implementations.

* Share callback between wait and fetch to address issue in which only one callback can be run for a given subscribe channel.

* Reactivate manager tests.

* Remove more code.

* Add some documentation.

lib/python/ray/worker.py

@@ -437,7 +437,7 @@ class Worker(object):
     Args:
       objectid (object_id.ObjectID): The object ID of the value to retrieve.
     """
-    self.plasma_client.fetch2([objectid.id()])
+    self.plasma_client.fetch([objectid.id()])
     buff = self.plasma_client.get(objectid.id())
     metadata = self.plasma_client.get_metadata(objectid.id())
     metadata_size = len(metadata)