Bluetooth: Mesh: Transport seg send timeout

Resolves a corner case where the segmented sending would be rescheduled
before the segments were done sending. This would happen if the
processing of each segment took longer than the retry delay. The
consequence of this was that some segments were resent multiple times at
the cost of other segments, occasionally causing a loss of transfers.

Rearranges, realigns and documents seg_tx structure bitfields in
transport.c. Removes bitwidth of counter fields, and collecting all flag
fields into one byte.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>