Availability after local scheduler failure (#329)

* Clean up plasma subscribers on EPIPE

First pass at a monitoring script - monitor can detect local scheduler death

Clean up task table upon local scheduler death in monitoring script

Don't schedule to dead local schedulers in global scheduler

Have global scheduler update the db clients table, monitor script cleans up state

Documentation

Monitor script should scan tables before beginning to read from subscription channel

Fix for python3

Redirect monitor output to redis logs, fix hanging in multinode tests

* Publish auxiliary addresses as part of db_client deletion notifications

* Fix test case?

* Small changes.

* Use SCAN instead of KEYS

* Address comments

* Address more comments

* Free redis module strings

python/ray/experimental/state.py

@@ -6,6 +6,8 @@ def get_local_schedulers(worker):
   local_schedulers = []
   for client in worker.redis_client.keys("CL:*"):
     client_info = worker.redis_client.hgetall(client)
+    if b"client_type" not in client_info:
+      continue
     if client_info[b"client_type"] == b"local_scheduler":
       local_schedulers.append(client_info)
   return local_schedulers