Track and use last known remote hash in pullv2

[ChrisPenner]

Overview

TODO: Need to ensure migration happens in create-schema

If we keep track of the last known hash of a remote branch according to the last time we synced with it, we can provide that hash to the server when running syncv2 and it knows which entities we'll need to receive by diffing the two.

Implementation notes

• Adds a migration to add the last_known_causal_hash to the remote branch table
• Sets the last known remote causal after each sync.
• Load and use this to skip causal-negotiation when running a V2 pull, if it's available (otherwise use causal negotiation).

Test coverage

• Need to test this more thoroughly