fix(pathfinder/sync/state_updates): fix state update sync

[kkovaacs]

This PR fixes a few unrelated issues in state update sync:

• querying the highest block with a state update should check nonces and class definitions in addition to storage updates
• counting declared classes for a block cannot use definition for the counting because unsynced classes have their definition set to NULL
• fix finding the start of the state update gap: now handles the case where there are no updates at all
• peer_agnostic::Client::state_diff_stream() should handle FIN as the stream terminator, not as a block delimiter