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/protocol/Feature.h

@@ -74,7 +74,7 @@ namespace detail {
 // Feature.cpp. Because it's only used to reserve storage, and determine how
 // large to make the FeatureBitset, it MAY be larger. It MUST NOT be less than
 // the actual number of amendments. A LogicError on startup will verify this.
-static constexpr std::size_t numFeatures = 54;
+static constexpr std::size_t numFeatures = 55;
 
 /** Amendments that this server supports and the default voting behavior.
    Whether they are enabled depends on the Rules defined in the validated
@@ -341,6 +341,7 @@ extern uint256 const fixTrustLinesToSelf;
 extern uint256 const fixRemoveNFTokenAutoTrustLine;
 extern uint256 const featureImmediateOfferKilled;
 extern uint256 const featureDisallowIncoming;
+extern uint256 const featureXRPFees;
 
 }  // namespace ripple