An alternative to the unity build, the classic build compiles each
translation unit individually. This adds more modules to the classic build:
* Remove unity header app.h
* Add missing includes as needed
* Remove obsolete NodeStore backend code
* Add app/, core/, crypto/, json/, net/, overlay/, peerfinder/ to classic build
* Added new test APIs allowing easy ways to create ledgers, apply
transactions to them, close and advance them.
* Moved Ledger tests from Ledger.cpp to Ledger.test.cpp.
* Changed several TransactionEngine log priorities from lsINFO to lsDEBUG to
reduce unnecessary verbosity in the log messages while running these tests.
* Moved LedgerConsensus:applyTransactions from a private member function to a
free function so that it could be accessed externally, and without having to
reference a LedgerConsensus object. This was done to facilitate the new
testing API.
