Fix: Mark block stake as withdrawn for the block

[daveroga]

Critical bug detected.
A proposer could call repeatedly the function requestBlockPayment in the Shield contract for the same block and will add BLOCK_STAKE to his pending withdrawal each time.

Adding state.setBlockStakeWithdrawn(blockHash) at the end of the function fixes this bug as it's checked in the function that the block stake is not already withdrawn for the blocHash.

require(
            state.isBlockStakeWithdrawn(blockHash) == false,
            'The block stake for this block is already claimed'
        );