[Chronik] Refactor: Add QueryBlocks, instead of using BlockReader directly

Summary: Encapsulates what we need to query blocks. This allows us to more easily add more data required for serving blocks, e.g. the current avalanche height.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13628