Use our actual feerate in open_channel messages, not a new one

When we were sending an open_channel messages we were asking the
feerate estimator for a new value instead of using the one we had.
If the feerate estimator gave a different value than the one it did
when we created the Channel struct, we'd start out-of-sync with our
counterparty and blow up on funding_signed. Even worse, the
ConfirmationTarget used was different, so its highly likely they
would disagree.

Also remove newly unused fee estimator parameter from get_open-channel
API.

Co-authored-by: Matt Corallo <git@bluematt.me>
Co-authored-by: Valentine Wallace <vwallace@protonmail.com>

Author: valentinewallace