MySQL handshake has a configurable version_string

MySQL handshake has no a configurable version_string parameter in
service section.
The default value is the server version of the embedded mysqld library.

Set option example:

version_string=5.5.37-MariaDB-log

Default is similar to:

5.5.35-MariaDB

server/include/service.h

@@ -38,6 +38,7 @@
  * 23/06/13	Mark Riddoch		Added service user and users
  * 06/02/14	Massimiliano Pinto	Added service flag for root user access
  * 25/02/14	Massimiliano Pinto	Added service refresh limit feature
+ * 07/05/14	Massimiliano Pinto	Added version_string field to service struct
  *
  * @endverbatim
  */
@@ -108,6 +109,7 @@ typedef struct service {
 			*router;		/**< The router we are using */
 	void		*router_instance;
 						/**< The router instance for this service */
+	char            *version_string;	/** version string for this service listeners */
 	struct server	*databases;		/**< The set of servers in the backend */
 	SERVICE_USER	credentials;		/**< The cedentials of the service user */	
 	SPINLOCK	spin;			/**< The service spinlock */