Listener and UDPConn are AsyncIterables instead of AsyncIterators.
The [Symbol.asyncIterator]()s are defined as generators and the
next() methods are gone.
"Listener/Socket has been closed" errors are now BadResource.
- Added `ServerRequest.finalize()`: consuming all unread body stream and trailers.
- This is cleanup method for reading next request from same keep-alive connection.
- Needed when handler didn't consume all body and trailers even after responding.
- refactor: `ServerRequest._bodyStream()`, `ServerRequestBody` are removed.
- Now using `bodyReader()` and `chunkedBodyReader()` instead.
- fix: Trailers should only be read `transfer-encoding` is `chunked` and `trailer` header is set and its value is valid.
- fix: use `Headers.append()` on reading trailers.
- fix: delete `trailer` field from headers after reading trailers.
- reorg: Several functions related to IO are moved into `http/io.ts`
