Classes implementing the consensus process on Ripple are cleaned
up in preparation for modularizations and compartmentalization.
Functions and state related to inter-round consensus are moved out
of NetworkOPs and into Consensus, where they are more effectively
isolated.
Some member functions are changed to free functions and some free
functions have their scope reduced to specific translation units.
* Track inter-round consensus state using new Consensus object
* Devirtualize interfaces
* Reduce NetworkOPs, Consensus and LedgerConsensus interfaces
* Add comments
* Inject dependencies, make functions free and levelize
* Add comments to document the intent behind the code
* Reduce class public interfaces
* Remove support for proposals without ledger hashes
