This command allows a user to increase the fee on a wallet transaction T, creating a "bumper" transaction B.
T must signal that it is BIP-125 replaceable.
T's change output is decremented to pay the additional fee. (B will not add inputs to T.)
T cannot have any descendant transactions.
Once B bumps T, neither T nor B's outputs can be spent until either T or (more likely) B is mined.
Includes code by @jonasschnelli and @ryanofsky
