multi: merge DetermineFeePerKw and Estimate

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.

htlcswitch/mock.go

@@ -81,6 +81,7 @@ func (m *mockPreimageCache) SubscribeUpdates(
 	return nil, nil
 }
 
+// TODO(yy): replace it with chainfee.MockEstimator.
 type mockFeeEstimator struct {
 	byteFeeIn chan chainfee.SatPerKWeight
 	relayFee  chan chainfee.SatPerKWeight