Refactor Ledger and support classes:

This performs a deep refactor on the Ledger class and its supporting
classes, in preparation for the move to shared_ptr<SLE const> in
places where the SLE is immutable and we are currently using
shared_ptr<SLE>.

Member functions are converted to free functions, the SLECache is an
explicit parameter, one line convenience functions are removed to
streamline the interface. Some callers are changed to use <SLE const>
instead of <SLE>

SLECache:
* Moved to its own header file

RippleState:
* Remove unused functions
* Store the SLE as const
* Simplify callers

AccountState:
* Remove unused members
* Simplify existing members

Ledger:
* Replace writeBack with insert and update
* Remove unused functions
* Remove LedgerStateParams
* Move getLastFullLedger to Application
* add entryCacheI, exists, fetch, erase
* Use boost::optional where it makes sense
* Make member functions free functions

Free functions:
* fetch: cache-aware SLE retrieval
* forEachItem, forEachItemAfter
* (various)

src/ripple/app/ledger/impl/LedgerCleaner.cpp

@@ -248,12 +248,14 @@ public:
         stopped();
     }
 
+    // VFALCO TODO This should return boost::optional<uint256>
     LedgerHash getLedgerHash(Ledger::pointer ledger, LedgerIndex index)
     {
-        LedgerHash hash;
+        boost::optional<LedgerHash> hash;
         try
         {
-            hash = ledger->getLedgerHash(index);
+            hash = hashOfSeq(*ledger, index,
+                getApp().getSLECache(), m_journal);
         }
         catch (SHAMapMissingNode &)
         {
@@ -262,7 +264,7 @@ public:
             getApp().getInboundLedgers().acquire (
                 ledger->getHash(), ledger->getLedgerSeq(), InboundLedger::fcGENERIC);
         }
-        return hash;
+        return hash ? *hash : zero; // kludge
     }
 
     /** Process a single ledger