Improve LevelDBStore

[cschuchardt88]

Change Log

• Added GetEnumerator to DB, Store and Snapshot
• Change UIntPtr/IntPtr to nuint/nint
• Change name from GetSnapshot to CreateSnapshot
• Changed name from NewIterator to CreateIterator
• Added DB unit tests

Type of change

• Optimization (the change is only an optimization)
• Style (the change is only a code style for better maintenance or standard purpose)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• TestLevelDb
• TestLevelDbDatabase
• TestLevelDbMultiSnapshot
• TestLevelDbSnapshot

Test Configuration:

• Unit Tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[nan01ab]

LGTM

[shargon]

I don't see any benefit of allow to iterate without a snapshot, but if it's ok for all except me, I'm not against to