fix: reference insertion without batch + verify

[fominok]

Issue being fixed or feature implemented

This PR addresses an issue with references insertion outside of a batch, also fixes verify function to align it all with batches and proof system

What was done?

Combined hash of references now uses value hash of the last item of references chain instead of the value hash of the direct item, which in turn could be a reference

How Has This Been Tested?

add tests for longer reference chains both for batch insertions and regular ones

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[QuantumExplorer]

Great!