kvdb+channeldb: create ExtendedBackend interface to allow rich tx logic

This commit adds the ExtendedBackend interface which is an extension to
the walletdb.DB interface. This paves the way to using etcd.db.View and
etcd.db.Update in the global View and Update functions without much code
rewrite.

channeldb/db.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcwallet/walletdb"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/channeldb/kvdb"
 	"github.com/lightningnetwork/lnd/channeldb/migration12"
@@ -147,12 +148,44 @@ var (
 // schedules, and reputation data.
 type DB struct {
 	kvdb.Backend
+
 	dbPath string
 	graph  *ChannelGraph
 	clock  clock.Clock
 	dryRun bool
 }
 
+// Update is a wrapper around walletdb.Update which calls into the extended
+// backend when available. This call is needed to be able to cast DB to
+// ExtendedBackend.
+func (db *DB) Update(f func(tx walletdb.ReadWriteTx) error) error {
+	if v, ok := db.Backend.(kvdb.ExtendedBackend); ok {
+		return v.Update(f)
+	}
+	return walletdb.Update(db, f)
+}
+
+// View is a wrapper around walletdb.View which calls into the extended
+// backend when available. This call is needed to be able to cast DB to
+// ExtendedBackend.
+func (db *DB) View(f func(tx walletdb.ReadTx) error) error {
+	if v, ok := db.Backend.(kvdb.ExtendedBackend); ok {
+		return v.View(f)
+	}
+
+	return walletdb.View(db, f)
+}
+
+// PrintStats calls into the extended backend if available. This call is needed
+// to be able to cast DB to ExtendedBackend.
+func (db *DB) PrintStats() string {
+	if v, ok := db.Backend.(kvdb.ExtendedBackend); ok {
+		return v.PrintStats()
+	}
+
+	return "unimplemented"
+}
+
 // Open opens or creates channeldb. Any necessary schemas migrations due
 // to updates will take place as necessary.
 // TODO(bhandras): deprecate this function.