Move TransactionStorage caches to a dedicated "index"

[jansegre]

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.

[jansegre]

This might still make sense.