Fixed connection release when body isn't run, as well as thread affinity

[djspiewak]

This PR fixes three issues in the async-http-client implementation:

• When the body of the response was never sequenced, the upstream connection would never be closed and would ultimately end up stuck in TCP CLOSE_WAIT
• The fiber continuation of the Response was run on an AHC thread, rather than on the pool governed by the relevant ContextShift. In order to avoid breaking bincompat, this was fixed by forking a fiber to run the async callback
• Under certain circumstances, when the body of an HTTP was empty, the prior implementation could deadlock. This is fixed by adding a callback to onComplete. Note that, by law, Cats Effect async callbacks ignore repeated invocations, so it's safe to put this here without a latch to guard it.

It's worth noting that there is still a known issue with this fix: namely, if you start running the body stream and then the controlling fiber is almost immediately canceled, then the connection can leak. This can happen because we cannot uninterruptibly call subscribe and start the stream (which would register the finalizer that runs cancel). The only way to fix this is to change fs2-reactive-streams' .stream function to take an F[Unit] representing the subscription effect, which it would then sequence and manage itself in a properly bracketed fashion.

This case is very unlikely, and at least things are better than they were.

[rossabaker]

Nice work.

Since this is the binary compatible fix, we should backport this to series/0.21.

[rossabaker]

Cherry-picked onto series/0.21.