Add missing Count api

impl/memory/datastore.go

Index: impl/memory/datastore.go
diff --git a/impl/memory/datastore.go b/impl/memory/datastore.go
index 62c9a99825b8b045c832d76958bc1c816d827563..e9d7a66d98008b01657dd5637a1a1d04cae0c5d2 100644
--- a/impl/memory/datastore.go
+++ b/impl/memory/datastore.go
@@ -68,6 +68,11 @@ func (d *dsImpl) Run(fq *ds.FinalizedQuery, cb ds.RawRunCB) error {
 	return executeQuery(fq, d.ns, false, idx, head, cb)
 }
 
+func (d *dsImpl) Count(fq *ds.FinalizedQuery) (ret int64, err error) {
+	idx, head := d.data.getQuerySnaps(!fq.EventuallyConsistent())
+	return countQuery(fq, d.ns, false, idx, head)
+}
+
 func (d *dsImpl) AddIndexes(idxs ...*ds.IndexDefinition) {
 	if len(idxs) == 0 {
 		return
@@ -145,6 +150,10 @@ func (d *txnDsImpl) Run(q *ds.FinalizedQuery, cb ds.RawRunCB) error {
 	return executeQuery(q, d.ns, true, d.data.snap, d.data.snap, cb)
 }
 
+func (d *txnDsImpl) Count(fq *ds.FinalizedQuery) (ret int64, err error) {
+	return countQuery(fq, d.ns, true, d.data.snap, d.data.snap)
+}
+
 func (*txnDsImpl) RunInTransaction(func(c context.Context) error, *ds.TransactionOptions) error {
 	return errors.New("datastore: nested transactions are not supported")
 }