Add option to create publishers latched with parameters

[esteve]

This PR provides a workaround for https://github.com/ros/actionlib/issues/32, while maintaining compatibility, so that starting action servers and clients don't need to be done sequentially. For example, a client can be created before a server and the server will pick up the request.

Publishers can be created as latched via ROS params, which default to false to maintain the current behavior.