Better cmd_vel command execution

[cst0]

There are some cases where the current interpretation of cmd_vels leaves much to be desired:

• If cmd_vel's are received at too low a rate, the robot does not move
• There's no acceleration/deceleration (at least not noticably), so jerking behavior is common
• There's some startup race condition where a series of 0'd cmd_vels can be sent, preventing locomotion (though this may be on the side of spot_ros_nav)