This is needed for a new validation of the source server.
The source server will 'ask' for the sha256 sum of files which are new or
modified and osc calculates the sha256 sums for those files and sends them
back to the server.
The server checks the sha256 sums and if dies if something is wrong.
missing log for a5a0917fce7124f9cc8458a9ee41ced521aa502e:
- added support for @POST, @PUT, @DELETE decorators
- support optional "exception" keyword arg for the decorators
(the specified exception will be raised before returning the response)
- moved "_check_digests" into the common module