NAKACK2: xmit_interval sometimes triggers unneeded retransmissions

In NAKACK2 (and UNICAST2 as well), a timer task kicks in every xmit_interval milliseconds. If we just added a message out of order, then the retransmit task might send a spurious retransmission message.
Example:

• Messages 1,2,3,4,5 are received at time T0 (in ms)
• At time T1000, message 20 is received
• At time T1001, the retransmit task kicks in and asks the sender for retransmission of messages [6-19]
• At time T1009, messages 6-19 are received

Problem: the out-of-order message 20 was received just before the retransmit task kicked in. Had the task waited for another 9 ms (until time T1009), the retransmission would not have been necessary.

The underlying cause is that the retransmit tasks handles recently added gaps the same as gaps added before xmit_interval, as we don't maintain a timestamp for added messages.

Another reason is also that the message bundler on the sender side might send messages in random order (see link below).

SOLUTION:
Investigate how to change the retransmission task such that it excludes gaps created within the last xmit_interval ms.