healthcheck: Send healthcheck results to subscribers only on updates

This MR changes hcmonitor behavior. Before this, HCMonitor would send every result received by the healthchecks to any subscriber interested. Now upon subscription, hcmonitor sends the latest result for each realserver and after that it sends updates on result change

To achieve this, a new field has been added to the Result struct and to the healtchecks: healthcheckID. It acts as a unique ID for each instance of a given healthcheck and it's currently implemented as jenkins(UUIDv7())

This MR ships commits from !39 (merged), !40 (merged), !41 (merged) and !42 (merged)