improve healthchecks so we can catch deadlocks easier
improve healthchecks so we can catch deadlocks easier

Summary:
This changes our health checks to schedule a simple task on the
update thread. We should be able to detect deadlock situations easier this
way. We could also schedule a task on the background thread, but this should
probably be sufficient.

Test Plan: built. Will canary before landing.

Reviewed By: tfang@fb.com

Subscribers: net-systems@

FB internal diff: D2255101

Tasks: 7622969

Signature: t1:2255101:1437193611:03623ee99a35810a14052945d9d8bee9e25e949a

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