Rework partial packet handling once more

Address the concern that commit c53c2fec raised differently.

The original direction of the traffic is encoded in bit 0
of the flight number.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5923)