MAJOR: checks: move health checks changes to set_server_check_status()

MAJOR: checks: move health checks changes to set_server_check_status()

We don't want to manipulate check's statuses anymore in functions
which modify the server's state. So since any check is forced to
call set_server_check_status() exactly once to report the result
of the check, it's the best place to update the check's health.