This allows players and the market maker to transact network sockets,
or other transport channels, by passing nonces, sigmaps, etc, back
and forth to each other. It also allows parties to store their signed
contract data locally, and enforce the contract even after a restart.
We were using btrees indexed by players, which required each
btree to store the player in memory. This was inefficient.
Instead, we now index by usize, which is a player index in the
sorted set of players in ContractParameters.
Each player only cares about the signatures which are needed
to enforce outcomes in which they receive money. Other outcomes
are entirely optional. The market maker must verify all signatures.
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.
