Add .trailers to Response/Request

Adds `Promise<Headers>` `trailers` properties to `Request` and
`Response`

When sending... `trailers` resolves to an empty, immutable
`Headers` since we're not dealing with sending trailers in
this change.

When receiving... `trailers` resolves to an immutable,
populated/filtered `Headers` resolved after the body
is completed. This means the trailers promise will not
resolve if the body is not consumed.

Author: jasnell