redeem process - add replay protection

[alexeiZamyatin]

Add replay protection to redeem protocol, to prevent a vault from re-using the same Bitcoin transaction as proof for multiple redeemRequests.

Require the following digest is included in OP_RETURN of the redeem Bitcoin transaction:

• redeemer Eth address
• contract address
• redeemRequestId

[alexeiZamyatin]

Requirements:

1) generate digest in requestRedeem. Emit raw data as event + return digest from function 2) verify digest is in OP_return in confirmRedeem function