MEDIUM: checks: simplify server up/down/nolb transitions

MEDIUM: checks: simplify server up/down/nolb transitions

We don't have to handle the maintenance transition here anymore so we
can simplify the functions and conditions. This also means that we don't
need the disable/enable functions but only a function to switch to each
new state.

It's worth mentionning that at this stage there are still confusions
between the server state and the checks states. For example, the health
check's state is adjusted from tracked servers changing state, while it
should not be.