Extract storage from blockchain construction

[tuommaki]

Description

This change separates storage instance construction from blockchain construction. This is necessary for having ability to use custom storage implementation as a backing storage for the blockchain.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Breaking changes

blockchain.NewBlockchain() takes now storage.Storage instance as a parameter, instead of dataDir describing path to storage (or empty string for in-memory one).

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[vcastellm]

LGTM 🎩