- replace File.data_sha1hex by File.data_hash
- update hooks accordingly
- store the hash with a "header" ("{SHA1}", "{MD5}" etc.)
- deprecate 'File.compute_sha1hex()' method
- add a 'File.compute_hash()' method
- add a 'File.hash_value' property
- add an helper 'File.check_hash()' method
- rename 'compute-sha1hex' config option as 'compute-hash'
- add a 'hash-algorithm' config option
- add a migration script