minimal async worker (#1410)
* async Worker._wait and its consequences
* AsyncPipe so that we can process idempotent endpoint and cancellation rather than _wait blocking the event loop
* test_prediction_cancel can be flaky on some machines
* separate _process_list to be less surprising than isasyncgen
* sleep wasn't needed
* suggestions from review
* suggestions from review
* even more suggestions from review
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Signed-off-by: technillogue <technillogue@gmail.com>
Co-authored-by: Nick Stenning <nick@whiteink.com>
Signed-off-by: technillogue <technillogue@gmail.com>
