Stopped nodes can rejoin immediately (#428)

* Ignore deleted clients when reading address info from Redis

* Remove self from db_client table when exiting cleanly

* Fix valgrind test

* Do not call plasma_perform_release when disconnecting

src/common/state/redis.cc

@@ -183,9 +183,21 @@ DBHandle *db_connect(const char *db_address,
 }
 
 void db_disconnect(DBHandle *db) {
+  /* Notify others that this client is disconnecting from Redis. If a client of
+   * the same type on the same node wants to reconnect again, they must
+   * reconnect and get assigned a different client ID. */
+  redisReply *reply =
+      (redisReply *) redisCommand(db->sync_context, "RAY.DISCONNECT %b",
+                                  db->client.id, sizeof(db->client.id));
+  CHECK(strcmp(reply->str, "OK") == 0);
+  freeReplyObject(reply);
+
+  /* Clean up the Redis connection state. */
   redisFree(db->sync_context);
   redisAsyncFree(db->context);
   redisAsyncFree(db->sub_context);
+
+  /* Clean up memory. */
   DBClientCacheEntry *e, *tmp;
   HASH_ITER(hh, db->db_client_cache, e, tmp) {
     free(e->addr);