Fix pathfinding with multiple issuers for one currency (RIPD-618).

* Allow pathfinding requests where the starting currency may have
  multiple issuers.

* Cache paths over all issuers to avoid repeating work.

* Clear the ledger checkpoint in one retry case.

* Add an additional node at the front of paths when the starting issuer
  is not the source account.
Remove unused isXRP(Issue) function.
Fix compilation errors in RPC/RipplePathFind.cpp
Adjust SSL context generation for Server:
The creation of self-signed certificates slows down the command
line client when launched repeatedly during unit test.
* Contexts are no longer generated for the command line client
* A port with no secure protocols generates an empty context
Add missing includes.
Refactor the structure of source files:
* New src/ripple/crypto and src/ripple/protocol directories
* Merged src/ripple/common into src/ripple/basics
* Move resource/api files up a level
* Add headers for "include what you use"
* Normalized include guards
* Renamed to JsonFields.h
* Remove obsolete files
* Remove net.h unity header
* Remove resource.h unity header
* Removed some deprecated unity includes