merkledb / sync -- Disambiguate no end root from no start root

[danlaine]

Why this should be merged

When a sync client asks a sync server for a change proof, and the server doesn't have sufficient history to generate and return a change proof, it falls back to trying to generate and return a range proof. However, we might find out while trying to generate the change proof that we lack sufficient history to return even a range proof. If this is the case, we shouldn't fall back to generating a range proof.

How this works

Add an additional ErrNoEndRoot type, which indicates that we lack sufficient history to generate even a range proof. If GetChangeProof returns ErrNoEndRoot, we immediately return drop the request instead of trying to generate a range proof.

How this was tested

Update existing UT.