Move TransactionStorage caches to a dedicated "index"

There are several properties in `TransactionStorage`, like `_cache_block_count`, `_cache_tx_count`, `_latest_timestamp`, `_first_timestamp` and maybe others, that could be in its own "index-like" implementation inside the IndexesManager, which would simplify some specialized code we had to make.

Start by reading TransactionStorage and IndexesManager to inventory the cache properties named in the issue, including _cache_block_count, _cache_tx_count, _latest_timestamp, and _first_timestamp. Determine which state belongs in a dedicated index-like implementation and how the existing specialized code depends on it. Done means the relevant caches are owned by that implementation and TransactionStorage is simplified without changing behavior.

Written by the indexing model from the issue text.