Finally, extends the 'close_to' functionality up to the flagship 'open a
channel' command.
Changelog-Added: JSON-API `fundchannel` now accepts an optional 'close_to' param, a bitcoin address that the channel funding should be sent to on close. Requires `opt_upfront_shutdownscript`
Fixes: #2679
Changelog-Added: JSON-RPC: New `multiwithdraw` command to batch multiple onchain sends in a single transaction. Note it shuffles inputs and outputs, does not use BIP69.