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/unity/app_paths.cpp

@@ -42,6 +42,4 @@
 #include <ripple/app/paths/cursor/ReverseLiquidityForAccount.cpp>
 #include <ripple/app/paths/cursor/RippleLiquidity.cpp>
 
-#include <ripple/app/paths/impl/PaymentView.cpp>
-
 #include <ripple/app/paths/tests/Path_test.cpp>