Further Cluster Metadata Storage Improvements

* MD5 prefixes for use in filenames. Raw binaries are unsafe and
  difficult to parse.
* Store metadata in in a manifest file. Folding over files in a directory
  to determine existing data is unsafe, error prone, and stupid.
* Re-allow prefixes to contain atoms. To prevent collision between binaries
  and atoms a filename trailer is appended.
* Sync all dets writes to disk so we don't easily lose data on emulator crash"".