BUG/MEDIUM: counters/server: fix server and proxy last_change mixup

BUG/MEDIUM: counters/server: fix server and proxy last_change mixup

16eb0fa ("MAJOR: counters: dispatch counters over thread groups")
introduced some bugs: as a result of improper copy paste during
COUNTERS_SHARED_LAST() macro introduction, some functions such as
srv_downtime() which used to make use of the server last_change variable
now use the proxy one, which doesn't make sense and will likely cause
unexpected logical errors/bugs.

Let's fix them all at once by properly pointing to the server last_change
variable when relevant.

No backport needed.