Take clients in a ID -> Client handle dictionary.

src/server.h

@@ -952,6 +952,7 @@ struct redisServer {
     list *clients_pending_write; /* There is to write or install handler. */
     list *slaves, *monitors;    /* List of slaves and MONITORs */
     client *current_client; /* Current client, only used on crash report */
+    rax *clients_index;         /* Active clients dictionary by client ID. */
     int clients_paused;         /* True if clients are currently paused */
     mstime_t clients_pause_end_time; /* Time when we undo clients_paused */
     char neterr[ANET_ERR_LEN];   /* Error buffer for anet.c */