[SERVER-9888] Add support for hashed sharding of the chunks collection with the filemd5 command.

Currently the 'filemd5' command will fail if the GridFS chunks collection is sharded using a hashed index on the files_id  field of the chunks.  The 'filemd5' capability is needed to be able to validate the contents of the GridFS volume and the use of a hashed shard key for the chunks ensure even distribution of  files (if not chunks).

The pull contains the changes needed to allow the use of the { 'files_id' : 'hashed' } shard key and the updates to the tests to validate the correct operation.