Check for converter for every message might be performance issue

[Karsten1987]

See here:
https://github.com/ros2/rosbag2/blob/master/rosbag2/src/rosbag2/sequential_reader.cpp#L66

For every message in the bag file there is going to be a check whether or not this is has to be converted. I believe this check can be done once per topic and an appropriate reader shall be used. Whenever the message has to be converted, I believe it makes sense to have a separate class for it which always converts. On the other side, when no conversion is needed (which is most likely almost all time the case), the check should not be issued for every message.

[anhosi]

I am not yet convinced that this line really poses a performance issue. Also our rather crude performance tests give no such indication (but they might not be sensitive enough).
More importantly having a single converter will no longer suffice with when #52 is implemented. Then we most likely will have a converter for each topic and every message will have to be dispatched to the converter for the corresponding topic.

We could think about having a "identity converter" for topics that require no conversion. Doing so before #52 is not necessary IMHO.

[emersonknapp]

We need to benchmark this to find out if it's necessary to change

[emersonknapp]

Best I can tell, this still exists and now refers to https://github.com/ros2/rosbag2/blob/master/rosbag2_cpp/src/rosbag2_cpp/readers/sequential_reader.cpp#L159

It seems to me that this simple check for nullptr is not of interest performance-wise, there are so many other things going on, that I am inclined to close this issue. @Karsten1987 if you think it's a problem, please reopen.