The server's open ledger is now an instance of the OpenView
class, managed by an instance of the OpenLedger class. This
should improve the performance of operations on open ledgers
because they are no longer Ledger/SHAMap operation.
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
