Maintain determinstic ordering of replica(s) in CLUSTER SLOTS response (

#265)

Sort `clusterNode.slaves` while adding a new replica to the cluster on
basis of `name`. This will enable deterministic ordering of replica(s)
information in `CLUSTER SLOTS` response.

Before this change:

```
127.0.0.1:6380> CLUSTER SLOTS
1) 1) (integer) 0
   2) (integer) 16383
   3) 1) "127.0.0.1"
      2) (integer) 6379
      3) "fc72609a620c62d073a31eed9ddde5104c1fa302"
      4) (empty array)
   4) 1) "127.0.0.1"
      2) (integer) 6381
      3) "fac84bbf2ffc5cfcdebc92c06b8ead9c3cba4051"
      4) (empty array)
   5) 1) "127.0.0.1"
      2) (integer) 6380
      3) "b1249d394326f1485df0b895f2fd38e141aa5056"
      4) (empty array)
```

After this change:

```
127.0.0.1:6380> CLUSTER SLOTS
1) 1) (integer) 0
   2) (integer) 16383
   3) 1) "127.0.0.1"
      2) (integer) 6379
      3) "fc72609a620c62d073a31eed9ddde5104c1fa302"
      4) (empty array)
   4) 1) "127.0.0.1"
      2) (integer) 6380
      3) "b1249d394326f1485df0b895f2fd38e141aa5056"
      4) (empty array)
   5) 1) "127.0.0.1"
      2) (integer) 6381
      3) "fac84bbf2ffc5cfcdebc92c06b8ead9c3cba4051"
      4) (empty array)
```

Signed-off-by: Harkrishn Patro <harkrisp@amazon.com>

src/cluster_legacy.c

@@ -1450,6 +1450,10 @@ int clusterNodeFailureReportsCount(clusterNode *node) {
     return listLength(node->fail_reports);
 }
 
+static int clusterNodeNameComparator(const void *node1, const void *node2) {
+    return strncasecmp((*(clusterNode **)node1)->name, (*(clusterNode **)node2)->name, CLUSTER_NAMELEN);
+}
+
 int clusterNodeRemoveSlave(clusterNode *master, clusterNode *slave) {
     int j;
 
@@ -1479,6 +1483,7 @@ int clusterNodeAddSlave(clusterNode *master, clusterNode *slave) {
         sizeof(clusterNode*)*(master->numslaves+1));
     master->slaves[master->numslaves] = slave;
     master->numslaves++;
+    qsort(master->slaves, master->numslaves, sizeof(clusterNode *), clusterNodeNameComparator);
     master->flags |= CLUSTER_NODE_MIGRATE_TO;
     return C_OK;
 }