Use constant sized arrays for some service strings

The `user`, `password`, `version_string` and `weightby` values should be
allocated as a part of the service structure. This allows them to be
modified at runtime without having to worry about memory allocation
problems.

Although this removes the problem of reallocation, it still does not make
the updating of the strings thread-safe. This can cause invalid values to
be read from the service strings.

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

@@ -248,7 +248,7 @@ int MySQLSendHandshake(DCB* dcb)
     GWBUF *buf;
 
     /* get the version string from service property if available*/
-    if (dcb->service->version_string != NULL)
+    if (dcb->service->version_string[0])
     {
         version_string = dcb->service->version_string;
         len_version_string = strlen(version_string);