ASACORE-776: Fix logic in TTL expiry on sender side

Update SND.UNA correctly when last unacked message expires.
Plus, fix condition for turning persist timer on/off.

Change-Id: Ib521d62994f154c5b3928a3a0317693a9c8c0390
Signed-off-by: Inga Stotland <ingas@qce.qualcomm.com>
Merge "ASACORE-776: Fix logic in TTL expiry on sender side" into feature/udp_transport