Unnecessary restriction on MassMigration commitments

[msieczko]

The current implementation of claimTokens method blacklists a pubkeyID for given withdrawRoot to prevent claiming the same token amount twice.

hubble-contracts/contracts/WithdrawManager.sol

Lines 81 to 84 in c22ce25

	require(
	!Bitmap.isClaimed(withdrawal.state.pubkeyID, bitmap[withdrawRoot]),
	"WithdrawManager: Token has been claimed"
	);

As a result this introduces a restriction on the MassMigration transactions that can be put into a single commitment. Namely, there can be no two transactions from UserStates with the same pubkeyID in a single MassMigration commitment.

Instead we could blacklist a specific path in withdraw states tree to drop this unnecessary restriction.