Revert "Removed _add_missing_downloads() from crawler's provider because missing downloads have been redownloaded"

This reverts commit 69f3b4b94ad986d9c5974a65de982d1a8b67c4b7.
Set the default values "read_file_downloads_from_gridfs" and "read_tar_downloads_from_gridfs" back to "no" because this shouldn't be the default behavior
Add a configuration property "read_gridfs_downloads_from_s3" for forcing reading GridFS downloads from S3
Integrate the _override_store_with_gridfs() helper back into the caller becaue we'll be adding support for more types of overrides
Add support for reading GridFS downloads from Amazon S3
Merge branch 'read_gridfs_downloads_from_s3'
Merge branch 'master' into RELEASE_20140325