MINOR: stats: report server's user-configured weight next to effective weight

MINOR: stats: report server's user-configured weight next to effective weight

The "weight" column on the stats page is somewhat confusing when using
slowstart becaue it reports the effective weight, without being really
explicit about it. In some situations the user-configured weight is more
relevant (especially with long slowstarts where it's important to know
if the configured weight is correct).

This adds a new uweight stat which reports a server's user-configured
weight, and in a backend it receives the sum of all servers' uweights.
In addition it adds the mention of "effective" in a few descriptions
for the "weight" column (help and doc).

As a result, the list of servers in a backend is now always scanned
when dumping the stats. But this is not a problem given that these
servers are already scanned anyway and for way heavier processing.