* 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
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