Make failsafe watchdog in standalone node.

[goldarte]

Request description
At the current time, there are 2 parts of "Raspberry failsafe" in client's software: delta position detection between setpoint and current position in copter_client.py and visual pose timestamp timeout detection in visual_pose_watchdog.py. In some situations these parts are triggered together and conflict with each other, e.g. when there is error in position and fcu switch to auto.land mode, after that visual_pose_watch try to decrease the motors power in offboard mode, but can't do it, because the fcu is already in auto.land mode.

Describe the solution you'd like
Make standalone node called failsafe.py, join all abilities there.

[goldarte]

Made here: c91bb94
But left the old name: visual_pose_watchdog