Check for idleness after the build is finalized instead of just complete
Fix "computer is gone" exception.

d73985c introduced clean up of offline slaves, but resulted in 'computer is gone' exceptions while connecting. This patch simplifies the appraoch by introducing a 'creationTime' for MansionComputers. This is used by MansionRetentionStrategy#shouldHaveConnectedByNow() to give some extra grace to computers which are taking a long time to connect.