XRPFees: Fee setting and handling improvements (#4247)

* Introduces amendment `XRPFees`
* Convert fee voting and protocol messages to use XRPAmounts
* Includes Validations, Change transactions, the "Fees" ledger object,
  and subscription messages

* Improve handling of 0 drop reference fee with TxQ. For use with networks that do not want to require fees
* Note that fee escalation logic is still in place, which may cause the
  open ledger fee to rise if the network is busy. 0 drop transactions
  will still queue, and fee escalation can be effectively disabled by
  modifying the configuration on all nodes

* Change default network reserves to match Mainnet

* Name the new SFields *Drops (not *XRP)
* Reserve SField IDs for Hooks

* Clarify comments explaining the ttFEE transaction field validation

src/ripple/rpc/impl/TransactionSign.cpp

@@ -720,8 +720,7 @@ checkFee(
         }
     }
 
-    // Default fee in fee units.
-    FeeUnit32 const feeDefault = config.TRANSACTION_FEE_BASE;
+    XRPAmount const feeDefault = config.FEE_DEFAULT;
 
     auto ledger = app.openLedger().current();
     // Administrative and identified endpoints are exempt from local fees.
@@ -738,11 +737,7 @@ checkFee(
 
     auto const limit = [&]() {
         // Scale fee units to drops:
-        auto const drops =
-            mulDiv(feeDefault, ledger->fees().base, ledger->fees().units);
-        if (!drops.first)
-            Throw<std::overflow_error>("mulDiv");
-        auto const result = mulDiv(drops.second, mult, div);
+        auto const result = mulDiv(feeDefault, mult, div);
         if (!result.first)
             Throw<std::overflow_error>("mulDiv");
         return result.second;