This commit adds a new expected field, `min_relay_feerate`, in the
response body returned from the API source, allowing the API to specify
a min relay feerate to be used instead of the FeePerKwFloor.
This change is backwards compatible as for an old API source which
doesn't specify the `min_relay_feerate`, it will be interpreted as zero.
This commit moves `DetermineFeePerKw` into the `Estimate` method on
`FeePreference`. A few callsites previously calling `DetermineFeePerKw`
without the max fee rate is now also temporarily fixed by forcing them
to use `Estimate` with the default sweeper max fee rate.