issueToken and registerCommit must only be called by same user account

[alexeiZamyatin]

Problem: if we do not check that the same user account is used to call issueToken, as registerCommit, the vault or a malicious user can call issueToken for arbitrary users with wrong data, causing it to failr (and collateral being slashed).

Solution: check that msg.sender is the same user that called registerCommit in issueToken