Refactor LedgerEntrySet:

* Remove duplicate:
    This changes behavior to fix an apparent bug. The
    copy will now correctly inherit mParams instead
    of reverting to tapNONE

* Tidy up LedgerEntrySet declarations
* Tidy up TransactionEngine
* Tidy PathCursor declarations
* Add LedgerEntrySet::apply
* Add LedgerEntrySet ctor
* Add Keylet, keylet namespace
* Add defaulted copy members
* Use optional in TransactionEngine
* Use optional<LedgerEntrySet> in PathState
* Return shared_ptr in Ledger::fetch
* Don't call entryCache with zero
* Deprecate invalidate
* Remove default constructor
* Remove unused container API
* Remove CountedObject base class
* Remove insert, clear
* Remove entryCreate overload
* Remove unused and tidy up STLedgerEntry
* Make getEntry private and tidy
* Replace members with adjustOwnerCount free function
* Replace accountFunds with funds free function

src/ripple/app/main/Application.cpp

@@ -1193,6 +1193,8 @@ bool ApplicationImp::loadOldLedger (
 
                              if (stp.object && (uIndex.isNonZero()))
                              {
+                                 // VFALCO TODO This is the only place that
+                                 //             constructor is used, try to remove it
                                  STLedgerEntry sle (*stp.object, uIndex);
                                  bool ok = loadLedger->addSLE (sle);
                                  if (!ok)