[xray] Implementing Gcs sharding (#2409)

Basically a re-implementation of #2281, with modifications of #2298 (A fix of #2334, for rebasing issues.).
[+] Implement sharding for gcs tables.
[+] Keep ClientTable and ErrorTable managed by the primary_shard. TaskTable is managed by the primary_shard for now, until a good hashing for tasks is implemented.
[+] Move AsyncGcsClient's initialization into Connect function.
[-] Move GetRedisShard and bool sharding from RedisContext's connect into AsyncGcsClient. This may make the interface cleaner.

src/local_scheduler/local_scheduler.cc

@@ -351,11 +351,6 @@ LocalSchedulerState *LocalSchedulerState_init(
     state->db = db_connect(std::string(redis_primary_addr), redis_primary_port,
                            "local_scheduler", node_ip_address, db_connect_args);
     db_attach(state->db, loop, false);
-
-    RAY_CHECK_OK(state->gcs_client.Connect(std::string(redis_primary_addr),
-                                           redis_primary_port, true));
-    RAY_CHECK_OK(state->gcs_client.context()->AttachToEventLoop(loop));
-    RAY_CHECK_OK(state->gcs_client.primary_context()->AttachToEventLoop(loop));
   } else {
     state->db = NULL;
   }

src/local_scheduler/local_scheduler_shared.h

@@ -60,8 +60,6 @@ struct LocalSchedulerState {
   std::unordered_map<ActorID, ActorMapEntry> actor_mapping;
   /** The handle to the database. */
   DBHandle *db;
-  /** The handle to the GCS (modern version of the above). */
-  ray::gcs::AsyncGcsClient gcs_client;
   /** The Plasma client. */
   plasma::PlasmaClient *plasma_conn;
   /** State for the scheduling algorithm. */