Restore old mission if a new mission is not feasible

[Igor-Misic]

Describe problem solved by this pull request
This solves the issue described here #12473

Describe your solution
Dataman has space to store 2 missions. This solution will keep one valid mission in Dataman while other space will be used to store new missions. If the new mission is not feasible, it will return to a previously feasible mission.
This solution also includes fixes that were raised when after this feature was implemented.

Test data / coverage
It is tested in simulation and real vehicles for the last 6 months.

[dagar]

What worries me about this solution is that the GCS is currently oblivious to the mission and has this assumption of being in sync.

Is this correct?

• the mission sync was technically a success, but then it's rejected by mission feasibility and the old mission is kept
• QGC is showing a mission that doesn't exist on the vehicle (and doesn't know it)
• if you click a waypoint on screen it's going to apply to a different mission

Let's at least pause to get a rough plan in place for resolving this fully in the near future.

[Igor-Misic]

This is correct, but it is not that bad and changes should probably be done on the GCS side.

Options:

1. GCS should be updated to download feasible missions from the vehicle after the new mission is rejected. (as it does now when it connects to vehicles that have existing mission)
2. GCS should have the capability to switch between two mission
3. GCS should do feasibility checks so it unfeasible mission will never be sent to the vehicle

In either way, I think it is better to have something so you can trigger RTL or LAND rather than have a vehicle in the air without a feasible mission.

Edit:
Since users need to pause the mission before updating the new one, I think the best solution will be number 1).
If GCS receives unfeasible mission information, when the user presses the button for the mission to continue, GCS should download the old mission

[Igor-Misic]

Closing this PR. In dev call was suggested a different approach where mission feasibility will be refactored to do validation more closely to mavlink level.

Everyone else who currently care more about the vehicle has a valid mission and to be capable of land and return to home, the branch is here: https://github.com/PX4/PX4-Autopilot/tree/pr-restore_old_mission_if_a_new_missionis_not_feasible