MINOR: backend: Add sample fetches to get the server's weight

The following sample fetches have been added :

 * srv_iweight : returns the initial server's weight
 * srv_uweight : returns the user-visible server's weight
 * srv_weight  : returns the current (or effetctive) server's weight

The requested server must be passed as argument, evnetually preceded by the
backend name. For instance :

  srv_weight(back-http/www1)
2 files changed