Feature Request: MAV_CMD_CONDITION

#### Issue details
It would really be nice if all 4 distance/time conditions existed:

- Time To
- Time From - already exists as MAV_CMD_CONDITION_DISTANCE
- Distance To - already exists as MAV_CMD_CONDITION_DELAY
- Distance From

There is currently no way to complete a mission command based on an estimated time TO a waypoint. This could be very useful for delivering packages without hovering. By using a time instead of distance estimate, the feature would inherently correct for head or tail wind for a drop.

#### Version
All

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

#### Airframe type
All

#### Hardware type
All

#### Logs
N/A

Start by tracing the existing MAV_CMD_CONDITION_DISTANCE and MAV_CMD_CONDITION_DELAY implementations across the Copter, Plane, and Rover mission-command paths. Define how an estimated time-to-waypoint condition should be represented and evaluated, then verify that missions can complete on that condition while the existing distance and delay conditions continue to work.

Written by the indexing model from the issue text.