IAVL Dependency Inversion Refactor - create nodeDB interface for immutable and mutable trees

[p0mvn]

Background

Please refer to #14 for more background information.

This task is specific to creating an interface for nodeDB and refactoring mutable and immutable tree to depend on the new abstraction instead of the concrete nodeDB struct.

Acceptance Criteria

• An interface for nodeDB is created
• mutable tree is refactored to depend on the new nodeDB interface
• immutable tree is refactored to depend on the new nodeDB interface
• mock nodedb is created and existing unit tests are refactored to use the mock
• existing unit tests are refactore
• additional unit tests are added where needed
• all unit tests pass

[rohanspersonalgit]

Hi, looking to contribute! working off my fork here, will appreciate any feedback on my progress as I get my PR ready.
https://github.com/RohansPersonalGit/iavl

[p0mvn]

thanks, @rohanspersonalgit . I saw a draft PR and will get back to you there soon

[p0mvn]

Given how risky changes on IAVL are. I think that this refactor is not worth the effort so I'm going to close this. Please let me know if any concerns