MXS-1343: send hostname to master with COM_REGISTER_SLAVE

A new option ‘slave_hostname’ allows the setting of hostname in
COM_REGISTER_SLAVE.

SHOW SLAVES HOSTS; in master server can show the hostname set in binlog
router:

MariaDB [(none)]> SHOW SLAVE HOSTS;
+-----------+-----------------------------+------+-----------+
| Server_id | Host                        | Port | Master_id |
+-----------+-----------------------------+------+-----------+
|        93 | maxscale-blr-1.mydomain.net | 8808 |     10124 |
+-----------+-----------------------------+------+-----------+

server/modules/routing/binlogrouter/blr.h

@@ -592,6 +592,7 @@ typedef struct router_instance
     int               master_semi_sync;     /*< Semi-Sync replication status of master server */
     BINLOG_ENCRYPTION_SETUP encryption;     /*< Binlog encryption setup */
     void              *encryption_ctx;      /*< Encryption context */
+    char              *set_slave_hostname;  /*< Send custom Hostname to Master */
     struct router_instance  *next;
 } ROUTER_INSTANCE;