[feature request] cross checks dependencies

Hi,

It would be really great if there was a way to connect checks in some sort of dependencies tree. It's especially important with integrations like PagerDuty or any other external services. It is really hard to imagine reason to send 10+ alerts to PagerDuty if 9+ alerts are result of the first one. Because kapacitor is triggering those alerts I think it's the right place to implement dependencies between them. It is possible right now to workaround that with an additional table in influxdb and store there information about alerts but it's a workaround. More native way for dependencies would be great.

Start by reviewing how Kapacitor triggers alerts and handles external services such as PagerDuty. Define how checks would be represented in a dependency tree and how downstream alerts should behave when an upstream alert fires. Done should provide a native dependency mechanism that avoids sending redundant alerts.

Written by the indexing model from the issue text.