Fill block time

[gennadylaventman]

We need block time added to block header and it should be monotonic increasing.

Protocol to calculate block time:

1. If current leader local time bigger that last committed block time, current leader sets its local time as block time for proposed block.
2. If current leader local time lower that last committed block time, leader will calculate difference, add minimal time unit to is and from this point on will set each proposed block block time as local time plus difference calculated earlier.