feat: gc: Mark snap sector key files for removal

[magik6k]

This PR adds additional marking logic to GC Mark task for cleaning up non-needed post-snap sector key files (original CC sector files):

Sectors where:

• sectors_meta table indicates a snap sector (orig_unsealed_cid != cur_sealed_cid)
• On-chain sector info is not null (1.5 finality ago)
• On-chain sector info has a non-nil SectorKeyCID indicating the sector is a snap sector
• On disk, the update and update-cache files are present for the sector, indicating that PoSt logic won't use the sealed file
• Have the sealed file on disk

Will have the sealed file (only) marked for GC

[LexLuthr]

Looks good.