feat(store): complete branch KVStore

[alexanderbez]

Description

Completes the branchkv.Store implementation, excluding tracing which will be done later in another PR.

The branchkv.Store will act as the main KVStore used in store v2. It handles branching and allows changesets to be accumulated which are flushed to SC and SS during Commit().

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• run make lint and make test
• confirmed all CI checks have passed

Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[github-actions]

@alexanderbez your pull request is missing a changelog!

[odeke-em]

Nice work and code wise LGTM, thank you @alexanderbez!