Reimplement tf_prefix in Noetic exactly as it was in Melodic

[sloretz]

This is an alternative to #139. The goal of this PR is to make ROS Noetic robot_state_publisher behave the same as the ROS Melodic version. It's different from #139 in these ways:

1. It calls the parameter tf_prefix
2. It reimplements the tf1 prefix and frame slash handling exactly as it behaves in ROS Melodic
3. It does not break ABI

While it doesn't break ABI, it effectively breaks using JointStateListener with a custom RobotStatePublisher. In this PR JointStateListener calls non-virtual versions of publishFixedTransforms() and publishTransforms(). Users who subclass RobotStatePublisher, override publishFixedTransforms() or publishTransforms(), and pass their subclass instance to a vanilla JointStateListener will observe their callbacks never being called.

Note: ROS 2 robot_state_publisher uses a different approach. It has a parameter frame_prefix that only appends a string to the frame name - no tf1 style special handling of slashes. See #159 for details.

[clalancette]

I've left one nit, but otherwise, this is a really good implementation. One other issue is that CI seems to be failing. I'm going to approve anyway, assuming that you'll fix both of these issues before merging.

[jmirabel]

Any chances this will be merged soon ?

It has been more than a year since the issue has been raised and proposal have been made. I think this functionality is useful and many people would love to use this again.

CI on focal fails because it failed to open the bag file. Does it ring a bell ?

[mlanighan]

Any updates on a timeline for when this will be merged?

[clalancette]

Looks good to me assuming CI goes green.