Use a single executor instance for spinning in client_async_callback. (backport #382)

[mergify]

In the current code, it calls rclpy.spin_once(), which instantiates a new executor, adds the node to it, executors one work item, then removes the node and destroys the executor.

Besides being inefficient, the problem with that sequence is that adding a node to the executor actually ends up being an "event", and so the work item that gets returned can be just the work of adding the node, over and over again. For reasons I admit I don't fully understand, this only happens with rmw_cyclonedds_cpp, not with rmw_fastrtps_cpp.

Regardless, the much more performant thing to do is to create an executor at the beginning of the example and to just spin on that. This eliminates adding it to the node constantly, and makes this work with all RMWs.

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This is an automatic backport of pull request #382 done by Mergify.

[mergify]

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	both modified:   rclpy/services/minimal_client/examples_rclpy_minimal_client/client_async_callback.py

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[ahcorde]

@clalancette probably we should backport this other one too https://github.com/ros2/examples/pull/379/files