Feature request

Feature description

Follow-up issue after #390. Prior to that PR, services' discovery would rely on built-in request/reply topics discovery, which can and did lead to races (see ros2/ros2#922). A short-term workaround was then introduced by #390, but not without caveats i.e. blocking behavior of rmw_send_response, non-standard (ab)use of sample identity data.

OMG DDS-RPC 1.0 spec describes both the issues associated with independent discovery of request/reply topics (see Basic Service Mapping under section 7.6.1) and an algorithm to prevent them (see Enhanced Service Mapping under section 7.6.2). Services' implementation in rmw_fastrtps*_cpp could and probably should move in that direction.