The `process-check-result` action can now optionally set the
`ttl` parameter. This overrules the configured freshness
check (check_interval).
The main idea behind this is to allow the external sender
to specify when the next check result is coming in.
For example, a backup script which should be run every
24h can specify the exact expected next check result.
The addition to the CheckResult class is necessary to
forward the check result throughout the cluster and
calculate the `next_check` value on each node. This
allows us to send in a check result on a satellite,
and the master determines the freshness and possible
notifications/state changes for Icinga Web 2.
