Remove unused callback stubs.

They were just overridden a bit further down anyway
Give callbacks the current task they're working on

Without this additional parameter, log messages are confusing. The task
variable used in log messages would end up referencing the one from the
containing frame, which in this case is always the last one in the
assignment.
Fix a few log messages
Fix misleading doc string

map_path() will _not_ communicate with the master.
Bugfix: Use correct config keys in JobType.node()
Bugfix: Don't apply timedelta if not set
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