issueToken: implement replay protection for BTC lock transaction

[alexeiZamyatin]

Problem: transaction can be replayed by a user to trick contract into issuing multiple tokens, possible with different contracts

Fix:

• Contract must check that contract address is in Bitcoin transaction -> prevent issuing on multiple issuing chains
• Contract generates a nonce which must be included in the Bitcoin tx -> counter

As a result, the contract returns a digest of:

• user eth address
• contract address
• counter