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/rpc/impl/Accounts.cpp

@@ -19,6 +19,7 @@
 
 #include <BeastConfig.h>
 #include <ripple/rpc/impl/Accounts.h>
+#include <ripple/app/main/Application.h>
 
 namespace ripple {
 namespace RPC {
@@ -40,7 +41,8 @@ Json::Value accounts (
 
         naAccount.setAccountPublic (naMasterGenerator, uIndex++);
 
-        AccountState::pointer as    = netOps.getAccountState (lrLedger, naAccount);
+        AccountState::pointer as    =
+            getAccountState (*lrLedger, naAccount, getApp().getSLECache());
 
         if (as)
         {