Refactor Ledger and LedgerEntrySet:

Member functions and free functions on Ledger and LedgerEntrySet are
rewritten in terms of new abstract interfaces `BasicView` and `View`,
representing the set of non-decomposable primitives necessary to read
and write state map items in a ledger, and to overlay a discardable
view onto a Ledger that can calculate metadata during transaction
processing. const-correctness is enforced through the parameter and
return types.

The MetaView now supports multi-level stacking: A MetaView can be
stacked on top of either a Ledger or another MetaView, up to any
number of levels.

The getSLEi member function is removed. The CachedView wrapper
replaces it, wrapping a View such that any function called with a
CachedView will go through the SLECache.

* Add BasicView, View, CachedView
* Rename LedgerEntrySet to MetaView
* Factor out free functions
* Consolidate free functions in ViewAPI
* Remove unused class members and free functions

src/ripple/rpc/impl/GetAccountObjects.cpp

@@ -39,7 +39,7 @@ getAccountObjects (Ledger const& ledger, AccountID const& account,
         found = true;
     }
 
-    auto dir = fetch(ledger, dirIndex,
+    auto dir = cachedRead(ledger, dirIndex,
         getApp().getSLECache(), ltDIR_NODE);
     if (! dir)
         return false;
@@ -62,7 +62,7 @@ getAccountObjects (Ledger const& ledger, AccountID const& account,
 
         for (; iter != entries.end (); ++iter)
         {
-            auto const sleNode = fetch(ledger, *iter,
+            auto const sleNode = cachedRead(ledger, *iter,
                 getApp().getSLECache());
             if (type == ltINVALID || sleNode->getType () == type)
             {
@@ -88,7 +88,7 @@ getAccountObjects (Ledger const& ledger, AccountID const& account,
             return true;
 
         dirIndex = getDirNodeIndex (rootDirIndex, nodeIndex);
-        dir = fetch(ledger, dirIndex,
+        dir = cachedRead(ledger, dirIndex,
             getApp().getSLECache(), ltDIR_NODE);
         if (! dir)
             return true;