change(doc): Simplify the database design using prefix iterators

[teor2345]

Motivation

To avoid storing the transaction count for each block, we can use RocksDB prefix iterators.

API Reference

API Function:
https://docs.rs/rocksdb/latest/rocksdb/struct.DBWithThreadMode.html#method.prefix_iterator_cf

API Docs:
https://github.com/facebook/rocksdb/wiki/Prefix-Seek#why-prefix-seek

Designs

Iterate through every transaction for a height, rather than storing a transaction count.

Solution

• Update the design to use a prefix iterator

Related changes:

• Make column family names consistent
• Fix some minor design details

Review

This is a low-priority design fix.

It would be good to have it merged by the database design review meeting later this week. (Wednesday evening UTC.)

Reviewer Checklist

• Design changes make sense

Follow Up Work

Implement the design in:

• Add lightwalletd full sync integration test #3511

[upbqdn]

Looks good, I have a few minor suggestions.