HTTP/3: allowed QUIC stream connection reuse.

HTTP/3: allowed QUIC stream connection reuse.

A QUIC stream connection is treated as reusable until first bytes of request
arrive, which is also when the request object is now allocated.  A connection
closed as a result of draining, is reset with the error code
H3_REQUEST_REJECTED.  Such behavior is allowed by quic-http-34:

   Once a request stream has been opened, the request MAY be cancelled
   by either endpoint. Clients cancel requests if the response is no
   longer of interest; servers cancel requests if they are unable to or
   choose not to respond.

   When the server cancels a request without performing any application
   processing, the request is considered "rejected."  The server SHOULD
   abort its response stream with the error code H3_REQUEST_REJECTED.

   The client can treat requests rejected by the server as though they had
   never been sent at all, thereby allowing them to be retried later.