Support reading request body

[bwindels]

Right now if the http handler read_headers method returns None, a 500 is returned from handle_no_response. Reading the body into a buffer (or file?) and calling read_body on the handler needs to be implemented still. read_headers should return whether the body should be read into a buffer or a file::Writer. This also ties into http handlers being able to wait on other async sources (like an http client) before responding. So read_headers would return either of:

• Ok(Response)
• Ok(ReadBody(File(size) | Buffer(size)))
• Ok(WaitingForOtherAsyncSources)
• Err

read_body would return:

• Ok(Response)
• Ok(WaitingForOtherAsyncSources)
• Err

a third method (handle_event) on the http handler would also be needed to handle non-connection-socket events, that would return the same as read_body:

• Ok(Response)
• Ok(WaitingForOtherAsyncSources)
• Err

One danger is that a bug could cause the response to never be returned if all 2 or 3 methods return WaitingForOtherAsyncSources

[bwindels]

it would be good if read_body or handle_event could not return ReadBody. So maybe read_headers returns:

enum HeadersResult {
  ReadBody(something that implements `BodyReader` or so)
  Respond(Option<Reponse>) //None means wait for more events
}

and read_body (and handle_event) keeps Result<Option<Response>> as a return type. If any of these methods return None as response, and there are no async sources registered apart from the connection socket, return 500.

[bwindels]

Requires #21

[bwindels]

investigate if we can somehow have a handler for each individual async source?