Smaller disk usage on start

At Fedimint we test against esplora as one of the support backends. This makes it part of our integration tests and "devimint" dev environment, and we spin up and down such environment frequently.

On start devimint environment would have less than 200 empty regtest blocks, yet:

```
15:49:11 …esplora/regtest/newindex
> du -cksh *
286M cache
286M history
286M txstore
858M total
```

That's kind of a lot for indexing... practically nothing yet. Is this some pre-allocation or is it supposed to be this way? If it's a tunable could there be (is there) an option to minimize it? In our test env we're not really concerned about raw throughput.