Replaces the existing calculation that uses a global online interval. This method was lacking due to the fact that different hosts may have different checkin intervals set.
The new calculation uses `min(distributed_interval, config_tls_refresh) + 30` as the interval. This is calculated with the stored values for each host.
Closes#1321
Notable refactoring:
- Use stdlib "context" in place of "golang.org/x/net/context"
- Go-kit no longer wraps errors, so we remove the unwrap in transport_error.go
- Use MakeHandler when setting up endpoint tests (fixes test bug caught during
this refactoring)
Closes#1411.
* Initial scaffolding of the host summary endpoint
* inmem datastore implementation of GenerateHostStatusStatistics
* HostSummary docstring
* changing the url of the host summary endpoint
* datastore tests for GenerateHostStatusStatistics
* MySQL datastore implementation of GenerateHostStatusStatistics
* <= and >= to catch exact time edge case
* removing clock interface method
* lowercase error wraps
* removin superfluous whitespace
* use updated_at
* adding a seen_at column to the hosts table
* moving the update of seen_time to the caller
* using db.Get instead of db.Select
