chore(sequencer): Move all storage keys to a single parent module (ENG-813)

[Fraser999]

Summary

This groups all keys used for DB entries to a single module, with two submodules, one for verifiable and the other for non-verifiable storage.

Background

DB keys must be unique, and it's easier to assess this when reviewing, updating or adding new keys if the keys are held in a single location.

Changes

• Consts and functions for generating DB keys were moved to storage::verifiable_keys or storage::nonverifiable_keys as appropriate. The const or generated value of every DB key should be unchanged by this PR.

Testing

• Existing snapshot tests were moved. New snapshot tests were added for all functions used to generate DB keys. The snapshot values in new .snap files which correspond to moved snapshot tests are unchanged.
• New tests added to ensure prefix keys are indeed prefixes of the relevant keys.

Related Issues

Closes #1494.