Use payment flow code for offer crossing (RIPD-1094):

Replace Taker.cpp with calls to the payment flow() code.

This change required a number of tweaks in the payment flow code.
These tweaks are conditionalized on whether or not offer crossing
is taking place.  The flag is explicitly passed as a parameter to
the flow code.

For testing, a class was added that identifies differences in the
contents of two PaymentSandboxes.  That code may be reusable in
the future.

None of the Taker offer crossing code is removed.  Both versions
of the code are co-resident to support an amendment cut-over.

The code that identifies differences between Taker and Flow offer
crossing is enabled by a feature.  That makes it easy to enable
or disable difference logging by changing the config file.  This
approach models what was done with the payment flow code.  The
differencing code should never be enabled on a production server.

Extensive offer crossing unit tests are added to examine and
verify the behavior of corner cases.  The tests are currently
configured to run against both Taker and Flow offer crossing.
This gives us confidence that most cases run identically and
some of the (few) differences in behavior are documented.

src/test/ledger/CashDiff_test.cpp

@@ -0,0 +1,104 @@
+//------------------------------------------------------------------------------
+/*
+  This file is part of rippled: https://github.com/ripple/rippled
+  Copyright (c) 2016 Ripple Labs Inc.
+
+  Permission to use, copy, modify, and/or distribute this software for any
+  purpose  with  or without fee is hereby granted, provided that the above
+  copyright notice and this permission notice appear in all copies.
+
+  THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+  WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+  MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+  ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+  WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+  ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include <BeastConfig.h>
+#include <ripple/ledger/CashDiff.h>
+#include <ripple/protocol/STAmount.h>
+#include <ripple/beast/unit_test.h>
+
+namespace ripple {
+namespace test {
+
+class CashDiff_test : public beast::unit_test::suite
+{
+
+    // Exercise diffIsDust (STAmount, STAmount)
+    void
+    testDust ()
+    {
+        testcase ("diffIsDust (STAmount, STAmount)");
+
+        Issue const usd (Currency (0x5553440000000000), AccountID (0x4985601));
+        Issue const usf (Currency (0x5553460000000000), AccountID (0x4985601));
+
+        // Positive and negative are never dust.
+        expect (!diffIsDust (STAmount{usd, 1}, STAmount{usd, -1}));
+
+        // Different issues are never dust.
+        expect (!diffIsDust (STAmount{usd, 1}, STAmount{usf, 1}));
+
+        // Native and non-native are never dust.
+        expect (!diffIsDust (STAmount{usd, 1}, STAmount{1}));
+
+        // Equal values are always dust.
+        expect (diffIsDust (STAmount{0}, STAmount{0}));
+        {
+            // Test IOU.
+            std::uint64_t oldProbe = 0;
+            std::uint64_t newProbe = 10;
+            std::uint8_t e10 = 1;
+            do
+            {
+                STAmount large (usd, newProbe + 1);
+                STAmount small (usd, newProbe);
+
+                expect (diffIsDust (large, small, e10));
+                expect (diffIsDust (large, small, e10+1) == (e10 > 13));
+
+                oldProbe = newProbe;
+                newProbe = oldProbe * 10;
+                e10 += 1;
+            } while (newProbe > oldProbe);
+        }
+        {
+            // Test XRP.
+            // A delta of 2 or less is always dust.
+            expect (diffIsDust (STAmount{2}, STAmount{0}));
+
+            std::uint64_t oldProbe = 0;
+            std::uint64_t newProbe = 10;
+            std::uint8_t e10 = 0;
+            do
+            {
+                // Differences of 2 of fewer drops are always treated as dust,
+                // so use a delta of 3.
+                STAmount large (newProbe + 3);
+                STAmount small (newProbe);
+
+                expect (diffIsDust (large, small, e10));
+                expect (diffIsDust (large, small, e10+1) == (e10 >= 20));
+
+                oldProbe = newProbe;
+                newProbe = oldProbe * 10;
+                e10 += 1;
+            } while (newProbe > oldProbe);
+        }
+    }
+
+public:
+    void run ()
+    {
+        testDust();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE (CashDiff, ledger, ripple);
+
+}  // test
+}  // ripple