Storage/remove memory store

[shaaibu7]

Summary

• Removed the in-memory storage backend, leaving Bolt DB as the sole storage.Store implementation.
• Updated tests to use temporary Bolt-backed stores; added helpers to create and clean up Bolt DBs.
• Fixed spectest helpers to pre-fill empty aggregated signature proofs, preventing nil dereferences during Bolt serialization.
• Adjusted spectest temp dir handling.