dnsdist: Be consistent with regard to health-check modes transition

dnsdist: Be consistent with regard to health-check modes transition

Calling `setAuto()` on a backend used to set the health-check mode
to `active`, even if it had been set to `lazy` before, which was
quite confusing.
This commit introduces a new method, `setAutoActive()` which can
be used to change the health-check mode to `active`, and alters the
behaviour of `setAuto()` to restore the previous health-check mode
instead. This is a breaking change but since the default health-check
mode is `active` I don't expect to break any existing configurations.
It also introduces a new method, `getHealthCheckMode()`, to inspect
the current mode.