WIP Refactor Transaction Proposals

WIP: DO NOT MERGE YET

- spawns TxProposal to a new class
- completely rewrite TxProposal and TxProposals tests to be really UNIT tests
- remove metadata from TxProposals. Remove PEER TRUST
- Adds multiple checks to received/read TxProposals:
  - must have a correct redeem script
  - signatures must be valid
  - metadata (signed and creator) is derived from the signature