multi: create channeldb/models package

Add a new subpackage to `lnd/channeldb` to hold some of the types that
are used in the package itself and in other packages that should not
depend on `channeldb`.

contractcourt/mock_htlcnotifier_test.go

@@ -1,11 +1,14 @@
 package contractcourt
 
-import "github.com/lightningnetwork/lnd/channeldb"
+import (
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channeldb/models"
+)
 
 type mockHTLCNotifier struct {
 	HtlcNotifier
 }
 
-func (m *mockHTLCNotifier) NotifyFinalHtlcEvent(key channeldb.CircuitKey,
+func (m *mockHTLCNotifier) NotifyFinalHtlcEvent(key models.CircuitKey,
 	info channeldb.FinalHtlcInfo) { //nolint:whitespace
 }