TLS: Connections refactoring and TLS support.

* Introduce a connection abstraction layer for all socket operations and
integrate it across the code base.
* Provide an optional TLS connections implementation based on OpenSSL.
* Pull a newer version of hiredis with TLS support.
* Tests, redis-cli updates for TLS support.

src/cluster.h

@@ -40,7 +40,7 @@ struct clusterNode;
 /* clusterLink encapsulates everything needed to talk with a remote node. */
 typedef struct clusterLink {
     mstime_t ctime;             /* Link creation time */
-    int fd;                     /* TCP socket file descriptor */
+    connection *conn;           /* Connection to remote node */
     sds sndbuf;                 /* Packet send buffer */
     sds rcvbuf;                 /* Packet reception buffer */
     struct clusterNode *node;   /* Node related to this link if any, or NULL */