send shutter "keep open" commands from pocs states

[fergusL]

At the moment the watchdog "keep open" commands are sent to the Musca shutter controller via a thread spawned in the init of the dome/shutter class. @AnthonyHorton has suggested it would be more robust to issue these commands from within a POCS state as then there will be no issues with lingering threads after POCS fails/shuts down.

The keep dome open command is issued here as part of the _async_dome_loop method that is run in the _dome_thread spawned here

Some additional things to consider pointed out by @AnthonyHorton:
"Might be a problem if taking long exposures though, longer than the watchdog timer period on Musca."