Fix connection leakage in dispatch and cursors

GODRIVER-397

Change-Id: Ia10bf4db82c78c85357437a0882bdccce499e0c2

Author: skriptble

core/dispatch/distinct.go

@@ -44,6 +44,7 @@ func Distinct(
 	if err != nil {
 		return result.Distinct{}, err
 	}
+	defer conn.Close()
 
 	return cmd.RoundTrip(ctx, desc, conn)
 }

core/topology/cursor.go

@@ -131,6 +131,7 @@ func (c *cursor) Close(ctx context.Context) error {
 
 	_, err = (&command.KillCursors{NS: c.namespace, IDs: []int64{c.id}}).RoundTrip(ctx, c.server.SelectedDescription(), conn)
 	if err != nil {
+		_ = conn.Close() // The command response error is more important here
 		return err
 	}
 
@@ -155,6 +156,7 @@ func (c *cursor) getMore(ctx context.Context) {
 
 	response, err := (&command.GetMore{ID: c.id, NS: c.namespace, Opts: c.opts}).RoundTrip(ctx, c.server.SelectedDescription(), conn)
 	if err != nil {
+		_ = conn.Close() // The command response error is more important here
 		c.err = err
 		return
 	}