BUG/MEDIUM: correctly disable servers tracking another disabled servers.

BUG/MEDIUM: correctly disable servers tracking another disabled servers.

In a config where server "s1" is marked disabled and "s2" tracks "s1",
s2 appears disabled on the stats but is still inserted into the LB farm
because the tracking is resolved too late in the configuration process.

We now resolve tracked servers before building LB maps and we also mark
the tracking server in maintenance mode, which previously was not done,
causing half of the issue.

Last point is that we also protect srv_is_usable() against electing a
server marked for maintenance. This is not absolutely needed but is a
safe choice and makes a lot of sense.

This fix must be backported to 1.4.