Currently, the proxy will retry requests with bodies if and only if they
have a `Content-Length` header with a value <= 64 KB. If the request has
a body but no `Content-Length` header, we currently assume that its body
will exceed the maximum buffered size, and skip trying to retry it.
However, this means gRPC requests will never be retried, because it
turns out gRPC requests don't include a `Content-Length` header (see
linkerd/linkerd2#7141). Whoops!

This PR fixes this by changing the retry logic so that `Content-Length`
is treated as a _hint_, rather than a requirement. Now, we will
preemptively skip retrying requests with bodies when the there is a
`Content-Length` header with a value that exceeds the limit, but if no
`Content-Length` header is present, we will still attempt to buffer the
body for a potential retry.

We are still protected against unbounded buffering because the buffering
body will stop buffering and discard any previously buffered data if the
buffered length ever exceeds the maximum. Requiring a `Content-Length`
simply allowed us to skip _trying_.

This should probably fix linkerd/linkerd2#7141, although it might be
worth testing with a non-Rust gRPC implementation to make sure it works?

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

`send_data` has to be awaited to actually...send the data, lol.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

using `let _` is how we ended up accidentally not awaiting the
`send_data` futures...we should probably also panic if the proxy dropped
the body too early, anyway.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

this changes the retry policy to check if the replay buffer was capped
on a previous iteration of the request. if it was, we will not retry the
request, allowing the original response to be propagated.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Currently, we look at each request's `content-length` header to
determine whether a request is too large to buffer for retries. This is
unnecessary, as hyper will already read the `content-length` and use it
populate the `Body::size_hint` value.

This change modifies the `ReplayBody` constructor (now
`ReplayBody::try_new`) to check the body's size hint and fails to
construct a `ReplayBody` if the lower bounds of the body size exceeds
the maximum buffer size.

In doing so, we update all `Body` implementations to ensure that
`size_hint` is properly implemenented. We also update the
`ReplayBody::size_hint` implementation to be more robust, returning the
original size hint if a clone of `ReplayBody` does not currently hold
the inner body.