omnibus actual concurrency and major refactor

[technillogue]

this should be reviewable at last. I'm mostly interested in whether the changes are comprehensible/legible and what comments I can add. there's a grab-bag of random changes like a /ready route, predictor.log, cancellation fixes, etc and the core change of moving worker into runner. I'm very open to reconsidering this (e.g. call it worker instead and maybe move some of it into http), but not right now. once this is merged, the plan is to gradually cut small changes from the async branch to merge into main, and review those changes more thoroughly.

original description

I originally tried to split up my work in #1499 and #1508 as "refactor runner + add concurrency" and "fix uploads/downloads" but ended up interleaving these changes. this PR will just be the overall changeset for now, and hopefully as this coalesces more it'll be clear how to carve it up into separate changesets

major points:

• add concurrency to cog.yaml
• use httpx for everything except URLFile, pull out all the client code from everywhere else
• completely rework URLPath
• very dirty hack to unblock us on large file uploads
• merge worker into runner (maybe the other way around would be better?)

[technillogue]

this should be reviewable at last. I'm mostly interested in whether the changes are comprehensible/legible and what comments I can add. there's a grab-bag of random changes like a /ready route, predictor.log, cancellation fixes, etc and the core change of moving worker into runner. I'm very open to reconsidering this (e.g. call it worker instead and maybe move some of it into http), but not right now. once this is merged, the plan is to gradually cut small changes from the async branch to merge into main, and review those changes more thoroughly.