Implement GC for PseudoKV

[jkawamoto]

More precisely, we need to implement this function:

us/renter/renterutil/metadb.go

Lines 401 to 403 in 080dbdd

	func (db *BoltMetaDB) UnreferencedSectors() (map[hostdb.HostPublicKey][]crypto.Hash, error) {
	return nil, nil // TODO
	}