Fixes issue #145

Provided a new interface `HealthcheckHandler` and deprecated `HealthcheckCallback`

This change is backward compatible i.e. the use of `HealthcheckCallback` stays as today.
We can get rid of this one and just use the new Provider<HealthCheckHandler>.  We can then just use HealthCheckCallbackToHandlerBridge as the default @ImplementedBy for HealthCheckHandler for now.