fix(p3): allow taking response/request body at most once

[rvolosatovs]

Similar to #156, specify that the request/response body can be taken only once.

If callers were able to get N references to the body, then they would be able to call body.finish N times, which would require multiplexing in the host to provide N copies of e.g. the trailers to these N references. It's hard to imagine a use case for this pattern and it also not a common pattern in HTTP libraries as far as I know, so specify that body can only be taken once.

[lukewagner]

Similar to my reply in #156, the idea is that a request/response can be handed to component A, it can call body to process a part of the body, and then drop the body handle, and then pass the request/response to component B, which will call body again, picking up where component A left off. That would suggest we add some verbiage to the same effect as on %stream where "you can only call this a second time after dropping the original body".