BUG/MINOR: server: don't miss server stats update on server state transitions

BUG/MINOR: server: don't miss server stats update on server state transitions

s->last_change and s->down_time updates were manually updated for each
effective server state change within srv_update_status().

This is rather error-prone, and as a result there were still some state
transitions that were not handled properly since at least 1.8.

ie:
- when transitionning from DRAIN to READY: downtime was updated
  (which is wrong since a server in DRAIN state should not be
   considered as DOWN)
- when transitionning from MAINT to READY: downtime was not updated
  (this can be easily seen in the html stats page)

To fix these all at once, and prevent similar bugs from being introduced,
we centralize the server last_change and down_time stats logic at the end
of srv_update_status():

If the server state changed during the call, then it means that
last_change must be updated, with a special case when changing from
STOPPED state which means the server was previously DOWN and thus
downtime should be updated.

This patch depends on:

- "MINOR: server: explicitly commit state change in srv_update_status()"

This could be backported to every stable versions.