Add StateDisabled for operator-initiated disable, keeping StateRemoved
for backends that disappear during a config reload. Previously both
used StateRemoved, which was confusing: "removed" implies the backend
no longer exists in config, but a disabled backend is still present
and can be re-enabled on the fly.
- health: add StateDisabled with String() "disabled", Disable() method
with probe code "disabled". Record() rejects probes in all three
inactive states (paused, disabled, removed).
- checker: DisableBackend calls backend.Disable() instead of Remove().
- docs: healthchecks.md rewritten for pause (goroutine cancelled, not
just results discarded), and separate disabled/removed state rows.
user-guide.md updated to match.
- Makefile: add fixstyle target (gofmt -w .).
- when a backend gets newly added or restarted, an event should fire; perhaps transition to 'unknown'
- when a backend gets removed, an event should fire; perhaps transition to 'removed'
- when a backend is in 'unknown' state, fast-interval is appropriate
