multi: add reset closure to kvdb.View

This commit adds a reset() closure to the kvdb.View function which will
be called before each retry (including the first) of the view
transaction. The reset() closure can be used to reset external state
(eg slices or maps) where the view closure puts intermediate results.

channeldb/meta.go

@@ -23,10 +23,12 @@ type Meta struct {
 // FetchMeta fetches the meta data from boltdb and returns filled meta
 // structure.
 func (d *DB) FetchMeta(tx kvdb.RTx) (*Meta, error) {
-	meta := &Meta{}
+	var meta *Meta
 
 	err := kvdb.View(d, func(tx kvdb.RTx) error {
 		return fetchMeta(meta, tx)
+	}, func() {
+		meta = &Meta{}
 	})
 	if err != nil {
 		return nil, err