Add server-side checksum selection for file writing.

Work in progress, too few tests. TODOs include:
* make sure no checksum is set on an OriginalFile without the corresponding hasher set
* make sure that raw file store writes first have the hasher set
* rename the sha1 property in OriginalFile and CheckedPath
* review and minimize references to,
  - ChecksumType.SHA1
  - ChecksumAlgorithmSHA1160
  - "SHA1-160"