[RPC] bumpfee

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

qa/pull-tester/rpc-tests.py

@@ -151,6 +151,7 @@ testScripts = [
     'signmessages.py',
     'nulldummy.py',
     'import-rescan.py',
+    'bumpfee.py',
     'rpcnamedargs.py',
 ]
 if ENABLE_ZMQ: