Fix unbonding period in deposit.sol

[DrZoltanFazekas]

The current deposit contract uses the block timestamp to determine if the unbonding period has expired and stakers can withdraw their deposit. This is vulnerable to the following attack. If the chain has a lot of timeouts (which can be caused by malicious validators) or, in an extreme case, it is down for a couple of days, the actual unbonding period in terms of blocks can become shorter than the weak-subjectivity period with the consequence that an adversary controlling 1/3 of the stake could escape slashing before the next checkpoint. Therefore, we should use block numbers instead of block timestamps.

The issue shall be fixed before the planned audit of the deposit contract,