hector_trajectory_server: -Wait for tf become available to suppress warning spam on startup

Sometimes ([1](http://answers.ros.org/question/40076/hector_slam-tutorial-errors/),[2](http://answers.ros.org/question/129891/mapping-trajectory-error/), see also #5) people get confused by hector_trajectory_server spitting out warnings (or errors in earlier versions) because tf data is not available during startup. This pull request adds a blocking wait for the required tf transform at the start, so a warning is not shown by default (only when more than 20s elapsed without tf data becoming available).