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.

src/common/redis_module/ray_redis_module.cc

@@ -8,22 +8,25 @@
 
 #include "common_protocol.h"
 
-/**
- * Various tables are maintained in redis:
- *
- * == OBJECT TABLE ==
- *
- * This consists of two parts:
- * - The object location table, indexed by OL:object_id, which is the set of
- *   plasma manager indices that have access to the object.
- * - The object info table, indexed by OI:object_id, which is a hashmap with key
- *   "hash" for the hash of the object and key "data_size" for the size of the
- *   object in bytes.
- *
- * == TASK TABLE ==
- *
- * TODO(pcm): Fill this out.
- */
+// Various tables are maintained in redis:
+//
+// == OBJECT TABLE ==
+//
+// This consists of two parts:
+// - The object location table, indexed by OL:object_id, which is the set of
+//   plasma manager indices that have access to the object.
+//   (In redis this is represented by a zset (sorted set).)
+//
+// - The object info table, indexed by OI:object_id, which is a hashmap of:
+//     "hash" -> the hash of the object,
+//     "data_size" -> the size of the object in bytes,
+//     "task" -> the task ID that generated this object.
+//     "is_put" -> 0 or 1.
+//
+// == TASK TABLE ==
+//
+// TODO(pcm): Fill this out.
+//
 
 #define OBJECT_INFO_PREFIX "OI:"
 #define OBJECT_LOCATION_PREFIX "OL:"