* plugin/forward: add HealthChecker interface

Make the HealthChecker interface and morph the current DNS health
checker into that interface.

Remove all whole bunch of method on Forward that didn't make sense.

This is done in preparation of adding a DoH client to forward - which
requires a completely different healthcheck implementation (and more,
but lets start here)
Signed-off-by: Miek Gieben <miek@miek.nl>

* Use protocol
Signed-off-by: Miek Gieben <miek@miek.nl>

* Dial doesnt need to be method an Forward either
Signed-off-by: Miek Gieben <miek@miek.nl>

* Address comments

Address various comments on the PR.
Signed-off-by: Miek Gieben <miek@miek.nl>