[RFE] configurable GRACE period before taking the next update

[miao0miao]

A user has automatic updates applied as soon as update_engine sees it, but they do not reboot the machines every 4 weeks or so. As it happens, the machine may be in a NEEDS_REBOOT state, while another update is already available. Immediately on reboot, the machine will begin to take that next update, thereby eliminating the chance of performing the rollback procedure.

A UPDATE_GRACE_PERIOD option would enforce that the next update would not occur for PERIOD after the last boot.