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/paths/PathRequest.cpp

@@ -149,8 +149,9 @@ bool PathRequest::isValid (RippleLineCache::ref crCache)
 
     if (bValid)
     {
-        auto asSrc = getApp().getOPs ().getAccountState (
-            crCache->getLedger(), raSrcAccount);
+        auto asSrc = getAccountState(
+            *crCache->getLedger(), raSrcAccount,
+                getApp().getSLECache());
 
         if (!asSrc)
         {
@@ -160,8 +161,8 @@ bool PathRequest::isValid (RippleLineCache::ref crCache)
         }
         else
         {
-            auto asDst = getApp().getOPs ().getAccountState (
-                lrLedger, raDstAccount);
+            auto asDst = getAccountState(*lrLedger,
+                raDstAccount, getApp().getSLECache());
 
             Json::Value& jvDestCur =
                     (jvStatus[jss::destination_currencies] = Json::arrayValue);
@@ -187,7 +188,7 @@ bool PathRequest::isValid (RippleLineCache::ref crCache)
             else
             {
                 bool const disallowXRP (
-                    asDst->peekSLE ().getFlags() & lsfDisallowXRP);
+                    asDst->sle().getFlags() & lsfDisallowXRP);
 
                 auto usDestCurrID = accountDestCurrencies (
                         raDstAccount, crCache, !disallowXRP);
@@ -196,7 +197,7 @@ bool PathRequest::isValid (RippleLineCache::ref crCache)
                     jvDestCur.append (to_string (currency));
 
                 jvStatus["destination_tag"] =
-                        (asDst->peekSLE ().getFlags () & lsfRequireDestTag)
+                        (asDst->sle().getFlags () & lsfRequireDestTag)
                         != 0;
             }
         }