Register logfile on master on starting a process
Expect "attempt" for tasks in assignments

The current attempt is needed when registering logs with tasks on the
master.
Remember log_identifier in ProcessData
Logfile names cannot include directories anymore

If log file names contain directories, that makes the logs much harder to
locate when they are requested via HTTP.
Bugfix: Forgot to import join
Consistency in callback argument order
Bugfix: Add referenced delay value to log call
Bugfix: Don't try to call a float
Upload log files to master after process ended
Don't return a DeferredList after all
Bugfix: Specify URL when delaying
Specify MIME-type when uploading task logs
Treat CREATED and ACCEPTED status codes as success
Bugfix: Open logfiles in binary mode

Text mode can lead to some automatic character conversions, which, if
UTF-8 is involved, can make the resulting data shorter than it was on
disk.  This would lead to the PUT request failing, because the sent data
is shorter than what was specified in the HTTP header.
Bugfix: Don't try to call a float (2)
Implement GET endpoint for tasklogs
Rename endpoint tasklogs to task_logs
Accept CONFLICT status code when registering log

A status code of CONFLICT from the server means that this log has
already been registered with this task attempt.
Accept CREATED status when registering log file
Remove unnecessary log call