Based on setchannelfee, but expanded to allow setting max htlc amount (and others
in future?).
The main differences:
1. It doesn't change values which are not specified (that would be hard to
add fields to!)
2. It says exactly what all values are in any potentially changed channels.
Changelog-Added: JSON-RPC: new `setchannel` command generalizes `setchannelfee`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
