Add OpenLedger:

The OpenLedger class encapsulates the functionality of
maintaining the open ledger. It uses an OpenView with the
last closed ledger as its base. Routines are provided to
modify the open ledger to add new transactions, and to
accept a new last closed ledger. Business logic for
performing transaction retries is rewritten to fit this
framework and used in the implementation of accept.

When the RIPPLE_OPEN_LEDGER macro is set to 1 (BeastConfig.h),
the global Application OpenLedger singleton maintains
its open ledger in parallel by applying new transactions
and accepting new last closed ledgers. In the current
implementation this does not affect transaction processing
but logs any differences in the results as compared to
the original code.

Logging shows an occasional mismatch in what the OpenLedger
builds versus the original code, usually an OfferCreate
which gets a terINSUF_RESERVE instead of tesSUCCESS.

src/ripple/ledger/View.h

@@ -117,6 +117,9 @@ public:
     using mapped_type =
         std::shared_ptr<SLE const>;
 
+    BasicView (BasicView const&) = delete;
+    BasicView& operator= (BasicView const&) = delete;
+
     BasicView()
         : txs(*this)
     {
@@ -643,6 +646,10 @@ operator&(ViewFlags const& lhs,
 class View : public BasicView
 {
 public:
+    View() = default;
+    View (View const&) = delete;
+    View& operator= (View const&) = delete;
+
     virtual ~View() = default;
 
     /** Returns the contextual tx processing flags.