API for improved Unit Testing (RIPD-432):

* 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.