In this commit, we introduce a new interface, the ContractResolver. The
duty of a ContractResolver is to watch a contract on-chain, for all
possible transitions, and exit finally when the contract has been fully
resolved. Resolvers themselves can be recursive: meaning producing
another resolver to hand off the duties require to fully resolve a
contract.
Each resolver also has a ResolverKit which contains all the function
closures and interfaces that the resolver need to properly do its job.
The 5 types of resolvers are:
* outgoing HTLC timeout
* outgoing HTLC contested
* incoming HTLC know presage
* incoming HTLC contested (don’t yet know)
* commitment sweep
In the future, more advanced resolver types can be added as required.
