Cover cases when CheckResult contains CheckData /w (checkingError != nil && checkData.LastSuccessfulCheckTimestamp == 0)

[kamaev]

https://github.com/moira-alert/moira/blob/4c03982d9e87e38354a5b648971cda6f580811f3/checker/check.go#L184

Currently used conditions leads to transitions into EXCEPTION state when: timeSinceLastSuccessfulCheck >= triggerChecker.ttl only because of timeSinceLastSuccessfulCheck = checkData.Timestamp - 0

and it's very unexpected to receive messages like "Remote server unavailable. Trigger is not checked for millions of years" for triggers created about 5 minutes ago (even less than TTL)