Track "source" for PatchSet

[LPGhatguy]

To prevent changes ping-ponging between clients, patch sets should have a source field attached to them. Each endpoint, including servers and clients, should generate a GUID source for themselves so that they can ignore changes that come from their own changes.

In tandem with #293, we should be able to track change sources for two-way sync and prevent clients from getting into funny states with themselves at least.