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.

lnwallet/chainfee/mocks.go

@@ -1,6 +1,8 @@
 package chainfee
 
-import "github.com/stretchr/testify/mock"
+import (
+	"github.com/stretchr/testify/mock"
+)
 
 type mockFeeSource struct {
 	mock.Mock
@@ -15,3 +17,50 @@ func (m *mockFeeSource) GetFeeMap() (map[uint32]uint32, error) {
 
 	return args.Get(0).(map[uint32]uint32), args.Error(1)
 }
+
+// MockEstimator implements the `Estimator` interface and is used by
+// other packages for mock testing.
+type MockEstimator struct {
+	mock.Mock
+}
+
+// Compile time assertion that MockEstimator implements Estimator.
+var _ Estimator = (*MockEstimator)(nil)
+
+// EstimateFeePerKW takes in a target for the number of blocks until an initial
+// confirmation and returns the estimated fee expressed in sat/kw.
+func (m *MockEstimator) EstimateFeePerKW(
+	numBlocks uint32) (SatPerKWeight, error) {
+
+	args := m.Called(numBlocks)
+
+	if args.Get(0) == nil {
+		return 0, args.Error(1)
+	}
+
+	return args.Get(0).(SatPerKWeight), args.Error(1)
+}
+
+// Start signals the Estimator to start any processes or goroutines it needs to
+// perform its duty.
+func (m *MockEstimator) Start() error {
+	args := m.Called()
+
+	return args.Error(0)
+}
+
+// Stop stops any spawned goroutines and cleans up the resources used by the
+// fee estimator.
+func (m *MockEstimator) Stop() error {
+	args := m.Called()
+
+	return args.Error(0)
+}
+
+// RelayFeePerKW returns the minimum fee rate required for transactions to be
+// relayed. This is also the basis for calculation of the dust limit.
+func (m *MockEstimator) RelayFeePerKW() SatPerKWeight {
+	args := m.Called()
+
+	return args.Get(0).(SatPerKWeight)
+}