Update FireFly smart contract(s) to be more extensible for the future

[awrichar]

Our current FireFly smart contract (provided for both Ethereum and Fabric) has exactly one method and one event, for recording a batch of FireFly messages.

Points raised in #804 and #820 have made it clear that this needs some expansion in order to cover more types of actions as the project evolves. Some changes have already been incorporated into these PRs, but more thought may be required to ensure the overall design is right for the next release.

This includes versioning the ABI of the contract itself (ie the method, event, and parameter naming), as well as versioning the rules of the network (such as the identity resolution changes in #804). It's possible that the rules should be able to change on the fly (without deploying a new contract), by allowing certain parties to call contract methods that advertise a rule change.

Throughout all of this, it should be noted that firefly-ethconnect (currently) only guarantees in-order delivery upon a single subscription - that is, a single named blockchain event. Therefore, near-term evolution of the contract should probably continue to include only one event (with parameters flexible enough to represent message batches or other network events).

[awrichar]

This is probably good enough for 1.1: #890