ros_realtime: Allow ignoring messages from a particular publisher

[ablasdel]

Migrated from code.ros.org, issue #2755
Reported by: sglaser

https://code.ros.org/trac/ros/ticket/2755

Consider a system for passing messages in realtime. ROS has various mutexes around publishing and subscribing, so to avoid blocking the messages must short-circuit ROS. It shouldn't avoid ROS completely: messages published should be received by other nodes, and other nodes should be able to send messages into realtime.

It's not currently possible to achieve this with ROS. Messages sent from a realtime publisher to a realtime subscirber will arrive twice: once from the short-circuited path and once from the standard ROS path. The realtime system should only pass on the message from the short-circuited path. To do this, it requires one of the following:

ROS must throw out messages from a particular publisher.
The message must be labeled with which publisher it comes from.
It's important that it be the publisher and not the node that distinguishes where the message came from. It's entirely possible to have a publisher and subscriber in the same process that do wish to communicate over standard ROS and not through the realtime short-circuited system.