Fix some edge cases with /sync (#302)

Including:
- Handle timeout=0 correctly
- Always return immediately on initial sync
- Handle spurious wake ups from the notifier

src/github.com/matrix-org/dendrite/syncapi/sync/notifier_test.go

@@ -258,7 +258,7 @@ func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 func waitForEvents(n *Notifier, req syncRequest) (types.StreamPosition, error) {
 	done := make(chan types.StreamPosition, 1)
 	go func() {
-		newPos := n.WaitForEvents(req)
+		newPos := n.WaitForEvents(req, req.since)
 		done <- newPos
 		close(done)
 	}()