Symbolically merging healthcheck changes from v0.16.x series (#2759)
* Also run release-specific CI for v0.16-maint branch
* Fix synchronous healthcheck
by managing via a separate channel & event loop.
This also removes `BasePredictor.healthcheck` since this makes detection
of a user-provided healthcheck method weirder and less reliable than it
needs to be, consolidates healthcheck code more into a single class,
removes now-unused `event_type` from `Done`, and attempts to generally
minimize the diff of total healthcheck code since v0.16.9.
* Shore up integration tests around healthcheck
* Remove skip lines in healthcheck integration tests
plus remove a duplicate and rename one of them.
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Co-authored-by: Nikhil Sinha <nsinha@cloudflare.com>