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/test/ledger/Invariants_test.cpp

@@ -64,7 +64,7 @@ class Invariants_test : public beast::unit_test::suite
             ov,
             tx,
             tesSUCCESS,
-            safe_cast<FeeUnit64>(env.current()->fees().units),
+            env.current()->fees().base,
             tapNONE,
             jlog};