sometimes get stuck (or extremely slow).

(https://github.com/grpc/grpc/issues/8545) created to debug poll-cv
issues with async_client_sync_server perf tests

These tests are failing when run as a batch, but passing when run in
isolation.

Suspect something more is up, but this will at least make our builds
much greener until we figure that out.

Delay beginning most writes until we enter poll()

Run those json configurations as part of presubmits