BUG/MINOR: quic: Possible stream truncations under heavy loss

BUG/MINOR: quic: Possible stream truncations under heavy loss

This may happen during retransmission of frames which can be splitted
(CRYPTO, or STREAM frames). One may have to split a frame to be
retransmitted due to the QUIC protocol properties (packet size limitation
and packet field encoding sizes). The remaining part of a frame which
cannot be retransmitted must be detached from the original frame it is
copied from. If not, when the really sent part will be acknowledged
the remaining part will be acknowledged too but not sent!

Must be backported to 2.7 and 2.6.