Address deadlock issue in client.call

[augustelalande]

Proposal to address the deadlock issue when calling client.call from within another callback.

I realise people may have different points of view on whether client.call should exist, and whether this proposal makes sense. But based on https://github.com/ros2/rclpy/pull/873#issuecomment-1008082009 ideas are welcome, so I thought I would at least propose it.