Summary:
Periodically ping client-sync's /health endpoint and store the latest
sync activity. If we get an ECONNREFUSED error, the worker window is
unavailable. Report the last known activity and restart the worker
window. Part of T7681.
Test Plan: manual, specs
Reviewers: evan, mark, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D4263
