More subscription modes

[nirinchev]

Currently, we only have a mode that returns the entire query when something changes. This is obviously inefficient and we can add different modes, such as getting only the changed objects back.

[pierre-moire]

Do you have alternative ways to handle partial updates? A global query refetch is a major drawback that make serious applications very slow: every time you change anything you have to wait for the server to resend the whole data tree.