commander: fix toggling datalink lost and regained
commander: fix toggling datalink lost and regained

When using QGC and/or the Dronecode SDK it was possible to get in a
state where the two mavlink instances were both publishing their last
heartbeat_time and cause commander to consistently toggle between
data link lost and regained. With this fix, we only ever look at the
very last heartbeat time and therefore seem to avoid this issue.

dagar/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,acfloria/Firmware,dagar/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware