HTLC Enhancements

[HagarMeir]

• [x] We have action Signature to carry the Claim Preimage. Action is reserved for tms actions (issue, transfer and setup). Signature itself is not an action. I see that having it this way allows for an easy lookup of preimages, but we could probably find another way, using TransferAction.GetMetadata() instead.
• [x] Review the terms of an exchange before accepting.
• [ ] How are these new operations tracked in the transaction db token/services/ttxdb. Recall that the auditors and each participants in the assembling of the transaction store traces in the transaction db.
• [x] Is the auditor able to audit the script? need code in integration/token/interop/views/auditor.go for that.
• [ ] How can the auditor track current holding when scripts are involved?
• [x] Is the acceptor able to inspect the script? maybe add code to integration/token/interop/views/htlc/lock.go.
• [x] ScanForPreImage actually works only when the backed is Fabric. But it should be backed agnostic by using the Network API (token/services/network).
• [ ] Extend all interop integration tests to orion