Wait the end of DO commands before finish mission

#### Issue details
When doing an auto mission, the mission stop when the last nav command is reach even if DO command are still running. (https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Mission/AP_Mission.cpp#L210)
The better logic should be to wait the end of the DO command if it already running,or finish the list of Do command in the mission

#### Version
Master

#### Platform
[X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover

Start in libraries/AP_Mission/AP_Mission.cpp around line 210 and trace how an auto mission decides it has finished after the final navigation command. Reproduce an auto mission with a still-running DO command, then verify that mission completion waits for the relevant DO work to finish or for the mission's DO-command list to be exhausted.

Written by the indexing model from the issue text.