Refactor View classes:

The View hierarchy of classes is reorganized to include new
classes with member functions moved and renamed, to solve
defects in the original design:

OpenView accumulates raw state and tx changes and
can be applied to the base. ApplyView accumulates changes
for a single transaction, including metadata, and can be
applied to an OpenView. The Sandbox allows changes with
the option to apply or throw them out. The PaymentSandbox
provides a sandbox with account credit deferral.

Call sites are changed to use the class appropriate for
the task.

src/ripple/app/tx/impl/apply.cpp

@@ -74,8 +74,8 @@ invoke (TxType type,
 }
 
 std::pair<TER, bool>
-apply (BasicView& view,
-    STTx const& tx, ViewFlags flags,
+apply (OpenView& view,
+    STTx const& tx, ApplyFlags flags,
         Config const& config,
             beast::Journal j)
 {