Stop video capture when no clients are active

[GeorgeHahn]

Pausing video capture while no clients are active would reduce power usage & heat production from the Pi's processor and the capture card.

This seems related to https://github.com/mtlynch/tinypilot/issues/317 since it also involves dynamically altering the stream.

[mtlynch]

Thanks for filing this!

I thought the current behavior was to pause video capture when no clients are connected, but I just tested it and was able to verify that it continues processing video on zero clients.

I'll look into how to remediate this.