refactor(http/retry): replayed body is not optional

[cratelyn]

ReplayBody<B> is slightly inconsistent about how it represents the exhaustion of the inner B-typed body.

first, in poll_data() we poll the inner body, and note that we consider a None value for BodyState::rest to mean that there is no more work to do.

// Poll the inner body for more data. If the body has ended, remember
// that so that future clones will not try polling it again (as
// described above).
let mut data = {
    // Get access to the initial body. If we don't have access to the
    // inner body, there's no more work to do.
    let rest = match state.rest.as_mut() {
	Some(rest) => rest,
	None => return Poll::Ready(None),
    };
    // ...
};

then, in Body::is_end_stream(), we write the following to check that the stream is done:

let is_inner_eos = self
    .state
    .as_ref()
    .and_then(|state| state.rest.as_ref().map(Body::is_end_stream))
    .unwrap_or(false);

note in particular the call to Option::and_then(..).

this means that, should the inner body be None, we could accidentally begin reporting the stream as not finished. this is backwards!

this commit makes a change to the definition of BodyState<B>. it no longer treats its inner body as optional. we never drop the inner body B, to facilitate gracefully replaying bodies that were interrupted when initially being polled.

a bare B means that various bits of control are now simpler, without having to account for the Option<T> state.

this is front-running other work to port the ReplayBody<B> to http-body 1.0. see linkerd/linkerd2#8733.