CLIENT LIST speedup via peerid caching + smart allocation.

This commit adds peer ID caching in the client structure plus an API
change and the use of sdsMakeRoomFor() in order to improve the
reallocation pattern to generate the CLIENT LIST output.

Both the changes account for a very significant speedup.

src/aof.c

@@ -507,6 +507,7 @@ struct redisClient *createFakeClient(void) {
     c->reply_bytes = 0;
     c->obuf_soft_limit_reached_time = 0;
     c->watched_keys = listCreate();
+    c->peerid = NULL;
     listSetFreeMethod(c->reply,decrRefCountVoid);
     listSetDupMethod(c->reply,dupClientReplyValue);
     initClientMultiState(c);