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/test/jtx/impl/utility.cpp

@@ -62,7 +62,7 @@ fill_fee (Json::Value& jv,
 {
     if (jv.isMember(jss::Fee))
         return;
-    jv[jss::Fee] = std::to_string(
+    jv[jss::Fee] = to_string(
         view.fees().base);
 }