Judgedaemon: add debug logging for API calls
Also base64 encode testcase files.

And update documentation to indicate which parameters are base64
allow_submit is not relevant (and not even SELECTed)
Make probid auto-incrementing numeric ID. Make string ID 'shortname'.

This makes it possible to have the same problem shortname in different
contests. It is (not yet) possible to assign one problem to multiple
contests.