MINOR: server: use functions to detect state changes and to update them

MINOR: server: use functions to detect state changes and to update them

Detecting that a server's status has changed is a bit messy, as well
as it is to commit the status changes. We'll have to add new conditions
soon and we'd better avoid to multiply the number of touched locations
with the high risk of forgetting them.

This commit introduces :
  - srv_lb_status_changed() to report if the status changed from the
    previously committed one ;
  - svr_lb_commit_status() to commit the current status

The function is now used by all load-balancing algorithms.