Automatic termination of cloud instance (including keep-alive)

[jeffi]

Is your feature request related to a problem? Please describe.
It is possible for users to accidentally leave a cloud instance running even after the robot's use of it is over. This would result in the user getting charged for time that they left the machine running. The current recommended practice is to check the AWS console to verify that all machines are terminated.

Describe the solution you'd like
FogROS should (1) automatically terminate a cloud instance when the robot side terminates, (2) if the cloud does not receive a termination notice (e.g., due to the robot dying or becoming disconnected), the cloud should detect inactivity and terminate instances with an user-specified timeout. The second part of this solution would require a keep-alive signal of some sort.

[mjd3]

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