Add step size to disk monitor

[lava]

📔 Description

Add a "step size" setting to the disk monitor to only trigger re-checking of the disk size after deleting N partitions. Useful for situations where computing the disk size is computationally expensive, e.g. on compressed filesystems.

📝 Checklist

• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

[dominiklohmann]

I just have three small comments and this needs a changelog entry. Other than that this is ready to ship.

I tested this locally with very small partition sizes and a custom check binary that counts partition files. It works very well.