[JENKINS-23676] Add logging and potential workaround for MansionComputers going offline during builds.
[JENKINS-23764] Use MansionComputer#onlineTime for more accurate idleness check.

Computer#connectTime is set when the computer starts connecting. If MansionRetentionStrategy is run immediately after MansionComputer completes connecting, the MansionComputer#getIdleStartMilliseconds() will reflect the time that it took to completely connect the MansionComputer instead of the time that it was actually idle.