Make Monitor remove dead Redis entries from exiting drivers. (#994)

* WIP: removing OL, OI, TT on client exit; no saving yet.

* ray_redis_module.cc: update header comment.

* Cleanup: just the removal.

* Reformat via yapf: use pep8 style instead of google.

* Checkpoint addressing comments (partially)

* Add 'b' marker before strings (py3 compat)

* Add MonitorTest.

* Use `isort` to sort imports.

* Remove some loggings

* Fix flake8 noqa marker runtest.py

* Try to separate tests out to monitor_test.py

* Rework cleanup algorithm: correct logic

* Extend tests to cover multi-shard cases

* Add some small comments and formatting changes.

python/ray/experimental/state.py

@@ -52,12 +52,18 @@ TASK_STATUS_MAPPING = {
 class GlobalState(object):
     """A class used to interface with the Ray control state.
 
+    # TODO(zongheng): In the future move this to use Ray's redis module in the
+    # backend to cut down on # of request RPCs.
+
     Attributes:
-      redis_client: The redis client used to query the redis server.
+        redis_client: The redis client used to query the redis server.
     """
     def __init__(self):
         """Create a GlobalState object."""
+        # The redis server storing metadata, such as function table, client
+        # table, log files, event logs, workers/actions info.
         self.redis_client = None
+        # A list of redis shards, storing the object table & task table.
         self.redis_clients = None
 
     def _check_connected(self):