Exclude servers with zero load from lag checks

Since r13582, servers with zero load have been isolated from the rest of
the servers in the section in terms of query flow. However, lag checks
still remained. This is not appropriate for backup/QueryPage servers
that are routinely lagged. IIRC we dealt with this once before with a
special case in the core excluding benet from master pos waits, but that
doesn't seem like a very appropriate solution today. Perhaps there is an
application for including zero-load slaves in lag checks, but I can't
think of any right now.

Related WMF conf change in I8fac5c99.

Change-Id: Id354039123f8c04febc8090eea9940fff044c5ea
Merge "Exclude servers with zero load from lag checks"