Auto merge of #7139 - samfoo:testable-net-load, r=jdm

Testable net load...

*The goal of this PR is to get early feedback on this before I go too far down the rabbit hole. This new code path is working, and there's several tests I've written as a proof of concept. There are still some regressions that I'll be fixing in the coming days.*

I've abstracted out the request/response cycle so that it's no longer dependent on the Hyper request/response structs. Since request/response @ hyper are structs, not traits, it made mocking them for tests impossible.

Current issues/concerns:

* This relies on boxing the `HttpResponse` that gets returned from the `HttpRequester` because `HttpResponse` is unsized. I don't know if there's a more idiomatic rust-y way of doing this?
* This relies on boxing the `Read` that is now returned from `load` for the same reason.
* The devtools and resource manager channels are still passed into `load`. It might be easier to inject these as trait dependencies instead of chans as well?
* Needs more tests.

:tophat: 

#6727

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