Use a single reentrant callback group for all subscriptions

[achim-k]

Public-Facing Changes

• Fix subscriptions not working on rolling

Description

• Callback groups that are created after the executor is already spinning do not work correctly, see Callback groups created after executor starts spinning do not work ros2/rclcpp#2067
• Instead of dynamically creating callback groups for each advertised channel, we now use a single reentrant callback group for all channels
  • Consequence is, that message callbacks of the same topic (e.g. when having multiple clients) can now be called in parallel. However, this is fine since the server::sendMessage is thread safe (also, callbacks of different topics were already called in parallel)

Fixes #94
Fixes #120