Use cluster for blocks and transactions

The records may be inserted in the order of the block number and many API returns the result in order of the block number.
In these cases, using cluster can make the insertion and reading fast.
https://www.postgresql.org/docs/9.1/sql-cluster.html

No files or tests are identified in the issue. Start by locating the block and transaction tables and the queries that return records by block number, then review PostgreSQL CLUSTER behavior and its operational requirements. Done means the relevant tables use clustering where appropriate and insertion and block-number-ordered reads are verified to improve.

索引モデルが issue の本文から書いたものです。