Reduce scope of RPC locks and general cleanup (RIPD-458)
Further documentation improvements to Ledger Consensus.
Use std::atomic
Make CBase58Data/RippleAddress movable (RIPD-428):
This significantly increases the performance in returning
these types from factory functions.
Large RippleCalc cleanup.

* Rationalize method and filenames, move to subdirectory.
* Use Issue in Node.
* Restrict access to PathState variables.
* Line length and readability cleanups.
* New PathCursor stores path calculation data during rippleCalc.
* Extract methods PathCursor::node(), PathCursor::previousNode()
  and RippleCalc::addPath