Add units to all fee calculations:

* Uses existing XRPAmount with units for drops, and a new TaggedFee for
  fee units (LoadFeeTrack), and fee levels (TxQ).
* Resolves #2451

src/ripple/basics/impl/mulDiv.cpp

@@ -36,7 +36,7 @@ mulDiv(std::uint64_t value, std::uint64_t mul, std::uint64_t div)
 
     result /= div;
 
-    auto const limit = std::numeric_limits<std::uint64_t>::max();
+    auto constexpr limit = std::numeric_limits<std::uint64_t>::max();
 
     if (result > limit)
         return { false, limit };