[MINOR] stats: add support for numeric IDs in set weight/get weight

[MINOR] stats: add support for numeric IDs in set weight/get weight

Krzysztof reported that using names only for get weight/set weight
was not enough because it's still possible to have multiple servers
with the same name (and my test config is one of those). He suggested
to be able to designate them by their unique numeric IDs by prefixing
the ID with a dash.

That way we can have :

     set weight #120/#2

as well as

     get weight static/srv1 10