Delete sectors that are no longer used

[ingar]

Description

There is a mechanism to mark a sector as "done" (filecoin-project/venus#2998), meaning that PoSts are no longer needed for it. While we can indicate this to the sector builder by simply excluding its CommR in the GeneratePostRequest, any previously sealed (but now unused) sectors still remain on disk, taking up space. For this functionality to be actually useful to miners, we need a way to clean up stale sectors and recover disk space.

Acceptance criteria

• A DeleteSector(sectorId uint64) (or similar) API, which will recover the storage space used by that sector.

Risks + pitfalls

Where to begin

[laser]

@icorderi - Any interest in taking this on?