mod+kvdb+channeldb: use btcwallet new DB interface

Depends on btcsuite/btcwallet#757.
Pulls in the updated version of btcwallet and walletdb that have the DB
interface enhanced by their own View() and Update() methods with the
reset callback/closure supported out of the box. That way the global
package-level View() and Update() functions now become pure redirects.

channeldb/reports.go

@@ -214,7 +214,7 @@ func (d DB) FetchChannelReports(chainHash chainhash.Hash,
 
 	var reports []*ResolverReport
 
-	if err := kvdb.View(d, func(tx kvdb.RTx) error {
+	if err := kvdb.View(d.Backend, func(tx kvdb.RTx) error {
 		chanBucket, err := fetchReportReadBucket(
 			tx, chainHash, outPoint,
 		)