sorted: make the in-memory implementation support Wiper

So an in-memory key/value store can be used as an index,
for the demo blob server on the website.

pkg/sorted/mem.go

@@ -31,10 +31,13 @@ import (
 // NewMemoryKeyValue returns a KeyValue implementation that's backed only
 // by memory. It's mostly useful for tests and development.
 func NewMemoryKeyValue() KeyValue {
-	db := memdb.New(comparer.DefaultComparer, 128)
-	return &memKeys{db: db}
+	kv := new(memKeys)
+	kv.Wipe()
+	return kv
 }
 
+var _ Wiper = (*memKeys)(nil)
+
 // memKeys is a naive in-memory implementation of KeyValue for test & development
 // purposes only.
 type memKeys struct {
@@ -42,6 +45,13 @@ type memKeys struct {
 	db *memdb.DB
 }
 
+func (s *memKeys) Wipe() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.db = memdb.New(comparer.DefaultComparer, 128)
+	return nil
+}
+
 // memIter converts from leveldb's iterator.Iterator interface, which
 // operates on []byte, to Perkeep's index.Iterator, which operates
 // on string.