Use the backend server charset

The default character set should be copied from the server so that
MaxScale appears to be the same. This fixes problems where utf8mb4
couldn't be taken into use because MaxScale would always send latin1 as
the server charset.

server/include/server.h

@@ -109,6 +109,7 @@ typedef struct server
     long           persistpoolmax; /**< Maximum size of persistent connections pool */
     long           persistmaxtime; /**< Maximum number of seconds connection can live */
     int            persistmax;     /**< Maximum pool size actually achieved since startup */
+    uint8_t        charset;        /**< Default server character set */
 #if defined(SS_DEBUG)
     skygw_chk_t    server_chk_tail;
 #endif