When t_poll_retransmit expires we need to consider an SDU for
retransmission.

We used to take the first SDU (which could be a segment, not a full SDU)
of the wait list and put it in the retransmit list.

When testing with some UE (Amarisoft UE) a bad behavior was detected.

This is what happens for the SDU with SN 6 (for illustration purpose).

The gNB is sending an SDU in several pieces (it is segmented).

slot n:     rlc sn 6 [1 .. 119[   p=0   RECEIVED
slot n+x:   rlc sn 6 ]120 .. 180[ p=0   RECEIVED
slot n+x+y: rlc sn 6 ]181 .. 210] p=1   not RECEIVED

Then when t_poll_retransmit expires rlc retransmits only the PDU of
slot n (with p=1 this time) and the UE replies with ack 6. So nothing
happens on the gnb side.
(We would have expected ack 7 + nack 6 181..end.)

Then after t_poll_retransmit expires again rlc retransmits only PDU
of slot n and the UE still replies with ack 6.

This goes on forever (or some other timeout occurs.)

The logic is now changed.

When t_poll_retransmit expires we transfer all the SDUs in the wait list
having the same SN as the head of the wait list into the retransmit list.

Testing with Amarisoft UE, it seems to work properly.