refactor(http/retry): use Poll::{map_ok, map_err}

[cratelyn]

this commit makes some small, admittedly subjective tweaks to ReplayBody::poll_trailers(..).

we can make use of the map_ok and map_err methods that exist to map Poll::Ready<Result<T, E>>'s inner T to some U (in our case, to record the trailers for later replay), and to map the error type into a boxed Error.

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