Current DAG is not updated if MFS root changes

[alanshaw]

When you start the explorer, it gets the CID of your MFS root and uses it as your "current working DAG". However if you change things in your MFS then that root CID will change and you'll be looking at out of date data.

I think the explorer should detect this by default and auto switch to the new CID, with a config option to turn it off.