BUG/MINOR: server: fix srv downtime calcul on starting

When a server is up after a failure, its downtime was reset to 0 on the
statistics. This is due to a wrong condition that causes srv.down_time
to never be set. Fix this by updating down_time each time the server is in
STARTING state.

Fixes the github issue #920.
This bug can be backported up to 1.8.
1 file changed