Alerting: Update scheduler to get updates only from database (#64635)
* stop using the scheduler's Update and Delete methods all communication must be via the database
* update scheduler's registry to calculate diff before re-setting the cache
* update fetcher to return the diff generated by registry
* update processTick to update rule eval routine if the rule was updated and it is not going to be evaluated at this tick.
* remove references to the scheduler from api package
* remove unused methods in the scheduler
(cherry picked from commit 85a954cd81)
# Conflicts:
# pkg/services/ngalert/api/api_ruler.go
# pkg/services/ngalert/ngalert.go
# pkg/services/ngalert/ngalert_test.go
# pkg/services/ngalert/schedule/schedule.go
# pkg/services/ngalert/schedule/schedule_mock.go
# pkg/services/ngalert/schedule/schedule_unit_test.go
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