Cluster: new sub-command CLUSTER COUNTKEYSINSLOT.

The new sub-command uses the new countKeysInSlot() API and allows a
cluster client to get the number of keys for a given hashslot.

src/cluster.c

@@ -1640,6 +1640,12 @@ void clusterCommand(redisClient *c) {
         sds key = c->argv[2]->ptr;
 
         addReplyLongLong(c,keyHashSlot(key,sdslen(key)));
+    } else if (!strcasecmp(c->argv[1]->ptr,"countkeysinslot") && c->argc == 3) {
+        long long slot;
+
+        if (getLongLongFromObjectOrReply(c,c->argv[2],&slot,NULL) != REDIS_OK)
+            return;
+        addReplyLongLong(c,countKeysInSlot(slot));
     } else if (!strcasecmp(c->argv[1]->ptr,"getkeysinslot") && c->argc == 4) {
         long long maxkeys, slot;
         unsigned int numkeys, j;