Clean up retry & long poll handling in retry client

[Sushisource]

There's a handful of ways calls can be considered long polls in the client code, by name or by (recently added) request extension. Make everything use extension instead of explicit name checking, and make sure metrics are set consistently for all long poll types. Probably also lift the code that always retries timeouts/cancels for task polling from the client into the worker poller so it's not special cased in the client.