Quorum polish and adding stopping state to migrationTool

[ChumpChief]

This change does a few cleanup items, mostly TODO items in the Quorum package:

• Removes has() from IQuorum. This was unreliable given that setting undefined would get cleaned up in summary -- clients would forget the key over time.
• Uses a TS generic for the quorum contents, similar to our other DDSs like map/cell
• Adds isPending() to enable distinguishing "nothing pending" from "pending delete/set(undefined)"
• Moves to using the more-accurate fluid-experimental scope rather than fluid-internal. Since we're still using the private: true flag it will continue to not publish.

It additionally adds a "stopping" state to the migration demo, mapping to the point where the proposal goes pending. This is the point where clients should stop writing new data into the app (as it will not be guaranteed to make it into the migration). This is how it always should have been, just an oversight on my part.

[github-actions]

This commit is queued for merging with the next branch! Please ignore this PR for now. Contact @microsoft/fluid-cr-infra for help.