feat/rocksdb: expose rocksdb config per LargeSmt tree

[drahnr]

As per discussion, lowers the default number of max open file descriptors to 64 from 512 per isntance and exposes them for host specific tuning.

There's a bit of boilerplate due to the conditional rocksdb feature gating.

Ref #1776

[Mirko-von-Leipzig]

I'm sad that a tech choice is leaking into our config and that there isn't a sensible default one can set and forget.

I wonder if we shouldn't instead have a single performance cfg low | high etc. Or maybe a warning log if ulimit is set too low..