Keep track of VNC sessions via UPID

[WillNilges]

The VNCProxy route gives back a UPID for a given task---including VMs
https://pve.proxmox.com/pve-docs/api-viewer/index.html#/nodes/{node}/qemu/{vmid}/vncproxy

And then you can use the API to monitor the status. Perhaps, if we store the UPIDs of the VNC sessions, then once per minute or so, we check the status of all of them (might have to throttle if the API gets sad), and then use that to remove Websockify targets.
https://pve.proxmox.com/pve-docs/api-viewer/index.html#/nodes/{node}/tasks/{upid}/status

[WillNilges]

Just a note for the future: We might be able to tie this into the redis work I'm doing in #158