Fix unfunded offer not removed (RIPD-1298):

If the mantissas of two non-native amounts differ by less than 10, then
subtracting them leaves a result of zero. This can cause situations
where `a>b`, yet `a-b == 0`.

One consequence of this is unfunded offers were incorrectly left in
order books. The code would check if the offer would be
consumed (`amount in offer > amount needed`), assume it wouldn't be,
yet when `amount needed` was subtracted from `amount in offer` the
result was zero and the offer was unfunded. This unfunded offer
incorrectly remained on the order book.

This patch fixes this bug.

src/ripple/ledger/impl/View.cpp

@@ -58,6 +58,20 @@ bool amendmentRIPD1274 (NetClock::time_point const closeTime)
     return closeTime > amendmentRIPD1274SoTime();
 }
 
+NetClock::time_point const& amendmentRIPD1298SoTime ()
+{
+    using namespace std::chrono_literals;
+    // TBD Move this date up when we know the release date
+    // Thu Jan 5, 2017 10:00:00am PDT
+    static NetClock::time_point const soTime{536954400s};
+    return soTime;
+}
+
+bool amendmentRIPD1298 (NetClock::time_point const closeTime)
+{
+    return closeTime > amendmentRIPD1298SoTime();
+}
+
 // VFALCO NOTE A copy of the other one for now
 /** Maximum number of entries in a directory page
     A change would be protocol-breaking.