AP_NavEKF: Rename static mode to avoid confusion with other non-GPS modes

This renames static mode as posHoldMode to make it clearer what this and the other non-GPS modes do
AP_NavEKF: Use compensation for baro delay  in position hold mode
AP_NavEKF: Relax timeout check applied to optical flow data

200 msec was too short and could lead to false positives. 5000 msec is the largest time we can go free inertial.