This adds additional states to the SigningSession state machine
which encapsulate the 2nd round of signing, and the final state
after signing is completed. Once all signatures have been received,
the Ticketed DLC is active and enforceable. Players can begin
buying ticket preimages from the market maker.
This is my first go at a higher level API which signers will
use to construct and sign the numerous transactions needed
for executing a Ticketed DLC. It revolves around the
SigningSession struct type, which has an API constrainted by
its current state.
