[FileRepo] Changed "publishBatch" to handle failure better.

[FileRepo] Changed "publishBatch" to handle failure better.

* Instead of moving the current file to the archive name,
  and then storing the new one, copy the current file to the
  archive name and overwrite the new one. When and object store
  backend is used, this reduces the number of operations from
  COPY, DELETE, PUT to just COPY and PUT. This reduces the RTTs,
  chances for failures, and avoids the period of time where the
  file has no current version.
* Also removed the "force" option to make file changes more likely
  to be all or nothing.

Change-Id: I46fc5c5c1fda5b386958b57557942f500de9dc2c