Add timestamps to rmw-internal structures.

[iluetkeb]

This goes together with ros2/rcl#591

[ralph-lange]

Connects to ros2/design#259.

[wjwwood]

I think we should (while we're at it) re-enable this field in the message info, as the main reason it was disabled was because we didn't know how to get the arrival timestamp in each case, which we will have to figure out for this:

rmw/rmw/include/rmw/types.h

Line 345 in 71b000c

// const rmw_time_t received_timestamp;

Also, feel free to suggest a different name if you think arrival_timestamp sounds better, or something similar.

Also, we should consider any changes to tests and documentation. One specific place I know is that we should mention this change in the release/migration notes for foxy:

https://github.com/ros2/ros2_documentation/blob/master/source/Releases/Release-Foxy-Fitzroy.rst#changes-since-the-eloquent-release

[iluetkeb]

Also, we should consider any changes to tests and documentation.

In principle, I would test that a) allocation and deallocation works and b) that the fields are filled properly.

However, it seems that a) happens in rcl and b) happens in the rmw implementation. So it doesn't appear as if I can test anything in rmw.

Suggestions to the contrary very welcome, I want this to be solid :-)

[wjwwood]

Thanks for iterating.

[iluetkeb]

Closed in favor of #214