Shard Redis. (#539)

* Implement sharding in the Ray core

* Single node Python modifications to do sharding

* Do the sharding in redis.cc

* Pipe num_redis_shards through start_ray.py and worker.py.

* Use multiple redis shards in multinode tests.

* first steps for sharding ray.global_state

* Fix problem in multinode docker test.

* fix runtest.py

* fix some tests

* fix redis shard startup

* fix redis sharding

* fix

* fix bug introduced by the map-iterator being consumed

* fix sharding bug

* shard event table

* update number of Redis clients to be 64K

* Fix object table tests by flushing shards in between unit tests

* Fix local scheduler tests

* Documentation

* Register shard locations in the primary shard

* Add plasma unit tests back to build

* lint

* lint and fix build

* Fix

* Address Robert's comments

* Refactor start_ray_processes to start Redis shard

* lint

* Fix global scheduler python tests

* Fix redis module test

* Fix plasma test

* Fix component failure test

* Fix local scheduler test

* Fix runtest.py

* Fix global scheduler test for python3

* Fix task_table_test_and_update bug, from actor task table submission race

* Fix jenkins tests.

* Retry Redis shard connections

* Fix test cases

* Convert database clients to DBClient struct

* Fix race condition when subscribing to db client table

* Remove unused lines, add APITest for sharded Ray

* Fix

* Fix memory leak

* Suppress ReconstructionTests output

* Suppress output for APITestSharded

* Reissue task table add/update commands if initial command does not publish to any subscribers.

* fix

* Fix linting.

* fix tests

* fix linting

* fix python test

* fix linting

src/common/common_protocol.h

@@ -5,6 +5,8 @@
 
 #include "common.h"
 
+#define DB_CLIENT_PREFIX "CL:"
+
 /**
  * Convert an object ID to a flatbuffer string.
  *