Technically there *are* two feerates that we need to know:
- the feerate to use for the funding transaction, and
- the feerate to tell our peer to use for our commitment txs/htlc txs
As written, `multifundchannel` uses the same feerate for both. This
optional parameter will allow us to differentiate between the two, which
will be exceedingly handy for anchor output worlds. ;)
FIXME: test this
Changelog-Added: JSON API: `multifundchannel` has a new optional argument, 'commitment_feerate', which can be used to differentiate between the funding feerate and the channel's initial commitment feerate
