Allow to use SmartRTL to get back on Mission track when leaved

#### Issue details
Actually smartrtl allow to get back to home.
In the case we are doing a mission in auto and for whatever reason we pause it, to go around or something like that, it would be really usefull to be able to use smartrtl code to get back safely on the last waypoint before continuing the mission or doing a failsafe strategie using Auto (coming with https://github.com/ArduPilot/ardupilot/pull/4723 that I am redoing)

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

Start by tracing the existing SmartRTL behavior and the Auto mission pause/resume flow; the issue does not name files or tests. Determine how the last mission waypoint should be selected after leaving the track, and define the expected behavior for safely returning before continuing or triggering an Auto failsafe strategy.

Written by the indexing model from the issue text.