Improve retransmission of message fragments. RFC 7383 states that loss of

Improve retransmission of message fragments.  RFC 7383 states that loss of
a single fragment results in a retransmit of all fragments belonging
to the same message.  Instead of treating each fragment as message with
seperate retransmit timer, keep only a single timer for all fragments of
a message and retransmit all fragments in order on timeout.
Improves reliability in case of packet loss when fragmentation is enabled.

Found by and diff from Daniel Herzinger
ok patrick@