JointGroupVelocityController: stop motion if no commands received

[rhaschke]

As a safety measure, this PR adapts the JointGroupVelocityController to reset control velocities to zero if no new command was received within a configurable interval (watchdog_timeout). If the steering process crashes or just finishes without resetting the velocity itself, the robot would otherwise just continue moving with the last speed commanded (and crash into something probably). I have chosen a rather large default timeout (1s) to limit interferences with existing code.

[wxmerkt]

Related issue and PR: #363 / #582

[rhaschke]

It's a pity that your initial PR wasn't merged yet, @wxmerkt. @bmagyar, how should we proceed with the two PRs addressing the same original issue?