fire zenduty alert when check fails 5 times in 5 mins

[ayazabbas]

• This further reduces the noise from zenduty by only alerting if a check fails 5 times within 5 minutes.
• Subsequently we alert every 5 minutes if it continues to fail at that rate (this does not produce a new message though, it gets collated under the same incident).
• Or if the check failed < 5 timees in the latest 5min window, the alert is resolved.
• This should exactly match the current behaviour of datadog alerts. I ran the program for a couple hours last night and alerts aligned with the datadog alerts.