Compare the hash of files with identical mtime/size #5589

Compare the hash of files with identical mtime/size #5589

* For conflicts where mtime and size are identical:

  a) If there's no remote checksum, skip (unchanged)
  b) If there's a remote checksum that's a useful hash, create a
     PropagateDownload job and compute the local hash. If the hashes
     are identical, don't download the file and just update metadata.

* Avoid exposing the existence of checksumTypeId beyond the database
  layer. This makes handling checksums easier in general because they
  can usually be treated as a single blob.

  This change was prompted by the difficulty of producing file_stat_t
  entries uniformly from PROPFINDs and the database.