Discard offline slaves if they aren't trying to connect

Previously, we looked at the offline cause to determine if the channel was terminated, but this doesn't include any other valid offline reasons. Mansion slaves are billed by the minute, so we guess that if a slave is offline the user does not want to be paying for it.
Only renew leases for computers that are online or connecting
[maven-release-plugin] prepare release mansion-cloud-1.18