MINOR: checks: simplify and improve reporting of state changes when using log-health...

MINOR: checks: simplify and improve reporting of state changes when using log-health-checks

Function set_server_check_status() is very weird. It is called at the
end of a check to update the server's state before the new state is even
calculated, and possibly to log status changes, only if the proxy has
"option log-health-checks" set.

In order to do so, it employs an exhaustive list of the combinations
which can lead to a state change, while in practice almost all of
them may simply be deduced from the change of check status. Better,
some changes of check status are currently not detected while they
can be very valuable (eg: changes between L4/L6/TOUT/HTTP 500 for
example).

The doc was updated to reflect this.

Also, a minor change was made to consider s->uweight and not s->eweight
as meaning "DRAIN" since eweight can be null without the DRAIN mode (eg:
throttle, NOLB, ...).