Fix deadlock with `rosbag play --clock --loop`

The short desc gives it all away:
If you loop a bag with /use_sim_time=true
you can (and regularly will) end up with a deadlock here:
All times published on /clock are before the expected timeout.
Merge pull request #31 from v4hn/fuerte-tf-no-waitForTransform-deadlock

Fix deadlock with `rosbag play --clock --loop`