Message listener for recording

[emersonknapp]

Description

Allow user to register a callback with the Recorder that will be called on every new recorded message. This need comes out of rqt_bag workflows. See #558 for full discussion.

Related Issues

Split from #558

Completion Criteria

• Add a user-provided callback to Recorder that can be called on every new message recorded
• Expose this API to Python client as well

Implementation Notes / Suggestions

** If you have ideas about how this feature might be accomplished, put them here. Note that this is just a suggestion to the implementer, so feel free to speculate. **

Testing Notes / Suggestions

** All features in this project need tests. Please give some input on cases that will need to be tested - and how the testing might be implemented. **

[MichaelOrlov]

@emersonknapp I am afraid that callback from each new message to the python API and then to the rqt_bag will hit performance of the entire recording process.
I think it would be better to gather needed statistics inside recorder and provide getters API to it. Perhaps we can use atomic variables without mutexes.
The idea is that rqt_bag will call statistic's getter API periodically instead of subscribing to the callbacks for each message write.

[emersonknapp]

Well, rqt_bag uses a much more inefficient method - when a message comes in, it triggers a separate thread to ask the actual open bagfile for messages in a timestamp range, and uses those results to populate the timeline. I'm not saying this is good, but a callback would be a strict improvement on running a read query on the bag that is being written, for every new message.

EDIT: note this is how ROS 1 version works - rosbag.Bag type supports making read queries into the bag while it is open for writing. rqt_bag calls Bag._get_entries in this case.