Disable protected mode in Sentinel mode.

Sentinel must be exposed, so protected mode is just an issue for users
in case Redis was started in Sentinel mode.

Related to redis#3279 and redis#3329.

src/sentinel.c

@@ -459,6 +459,7 @@ struct redisCommand sentinelcmds[] = {
  * specific defaults. */
 void initSentinelConfig(void) {
     server.port = REDIS_SENTINEL_PORT;
+    server.protected_mode = 0; /* Sentinel must be exposed. */
 }
 
 /* Perform the Sentinel mode initialization. */