Refactor/extract block store from blocktree

[asdacap]

• Extract encoding/decoding and caching logic of blocks from blocktree to its own class.
• Make it easier to implement other block storage related logic.

Changes

• Extract encoding/decoding and caching logic of blocks from blocktree to its own class.
• Remove an unused method.

Types of changes

What types of changes does your code introduce?

• Refactoring

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No