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To reduce future import cycles, and expose more types properly, move the root package to an
interface/types file and move the main implementation into /impl. This also exposes an interface
other implementers can replace.

Move ResponseProgress to the types package and extract some testing functions to testutil

make sure order channels are consumed in repsonse collector doesn't block sends on the other

Make the request manager actually inject responses from the network, and perform a selector
verification

Add a channel of asynchronous errors instead of only an initial synchronous one

Create a manager for outgoing requests