Allocate part of block reward to pay Validators

[Gnappuraz]

Validators need to be rewarded for their effort in maintaining the blockchain secure. We allocate then part of the block reward not to be minted immediately but instead to be given to Validators as a reward.

To achieve that need to:

• define a new parameter that defines how much of the block reward is reserved.
• show (with a test) that we don't print more than what we reserve in the case all validators withdraw.

The validation logic to check if the amount withdrawn is correct should already be in place in ValidateWithdraw().