Proposal: Remove siapath from siafile metadata
This MR is a proposal to remove the siapath field from the siafile metadata and to use the location of the siafile and its name to determine the siapath.
This solves an issue for siafile backups where inconsistencies could happen if a siafile
foo.sia is restored as
foo_1.sia but it still contains the siapath
foo in the metadata.