Change server version from char pointer to char array

With this change, it is no longer possible that the server version
is deallocated at the very moment it is read. There is still a race,
but it's mostly harmless.

server/modules/protocol/MySQL/MySQLClient/mysql_client.c

@@ -224,8 +224,7 @@ int MySQLSendHandshake(DCB* dcb)
     {
         mysql_server_language = dcb->service->dbref->server->charset;
 
-        if (dcb->service->dbref->server->server_string &&
-            strstr(dcb->service->dbref->server->server_string, "10.2."))
+        if (strstr(dcb->service->dbref->server->version_string, "10.2."))
         {
             /** The backend servers support the extended capabilities */
             is_maria = true;