Prepare getKeepAliveTime method for MCAccess.

No implementation of it yet. If implemented this will help finding
players that do a passive god-mode.
Add workaround method for keep-alive time, as a temporary solution.

Also keep track of last-regain-health time for that and other purpose.
Use the keep-alive estimation for the new god-mode check.

Might not fully do it, but should make occasional hits possible.