The necessity of SOstart/SOend was understood after email exchanges with
David Breaux <davidbreaux67@gmail.com>.

See openair2/LAYER2/rlc_v2/tests/test46.h for some details.

Basically if we NACK without SOstart/SOend then the other end of the RLC
channel will consider all the bytes as NACKed and will have to resend them
all. If another status is then reported before all the bytes are sent,
and because we don't use SOstart/SOend this status will NACK all the bytes
again, then all the bytes are to be sent again. And the cycle repeats.
It will end with a max retx reached (which triggers RLF, Radio Link Failure).

The function put_bit in openair2/LAYER2/rlc_v2/rlc_pdu.c has been changed
too, maybe introducing some performance issues.