Single threaded executor for multiple nodes

[sayali-purdue]

In each of the following files minimal_lock_free_queue.cpp and minimal_sched_fifo_callback_group.cpp, an executor with a single thread is used to process callbacks of two nodes, which means a single thread is responsible to process callbacks of multiple callback groups. As per the ROS documentation, callbacks from different groups can execute in parallel. So, Using a multi-threaded executor would be more efficient for this use case.

Would appreciate if you could share your thoughts/reasoning on this.