[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

src/rpc/client.cpp

@@ -116,6 +116,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "setnetworkactive", 0, "state" },
     { "getmempoolancestors", 1, "verbose" },
     { "getmempooldescendants", 1, "verbose" },
+    { "bumpfee", 1, "options" },
     // Echo with conversion (For testing only)
     { "echojson", 0, "arg0" },
     { "echojson", 1, "arg1" },