Break up sync responses

[a-type]

When resyncing a whole library, it's kind of a lot. Batching responses by some reasonable number of operations / baselines would probably be warranted. A theresMore kind of flag could tell the client to keep waiting.

[a-type]

NOTE: when doing this, the sync layer should merge all responses before passing back to client. The client is not designed for multiple sync responses, particularly because of old version handling with #413