Cleanup stale device lists for users we don't share a room with anymore (#2857)

The stale device lists table might contain entries for users we don't
share a room with anymore. This now asks the roomserver about left users
and removes those entries from the table.

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

roomserver/api/query.go

@@ -447,3 +447,15 @@ type QueryMembershipAtEventResponse struct {
 	// do not have known state will return an empty array here.
 	Memberships map[string][]*gomatrixserverlib.HeaderedEvent `json:"memberships"`
 }
+
+// QueryLeftUsersRequest is a request to calculate users that we (the server) don't share a
+// a room with anymore. This is used to cleanup stale device list entries, where we would
+// otherwise keep on trying to get device lists.
+type QueryLeftUsersRequest struct {
+	StaleDeviceListUsers []string `json:"user_ids"`
+}
+
+// QueryLeftUsersResponse is the response to QueryLeftUsersRequest.
+type QueryLeftUsersResponse struct {
+	LeftUsers []string `json:"user_ids"`
+}