[Next] simplify AsyncTask code and use RsyncTask to estimate Snapshot size

[ygerlach]

This continues what i started with #427.
This does not mean #427 is obsolete. #427 still helps to combat a race condition.

What did i do here?

• simplified code for all classes related to AsyncTask
  • remove unused (never set to true) AsyncTask.is_terminated
  • remove unused (never set) AsyncTask.log_file
  • remove unused function read_status from Tasks
  • remove other unused functions and members from AsyncTask
  • move content of finish_task to AsyncTask (its code was already contained inside finish)
  • make prepare virtual and move base implementation to AsyncTask
  • move execute to AsyncTask
  • simplify closing of streams
• make RsyncTask.regex_list an array (this avoids stupid typo bugs and i guess its a little not noticeable performance improvement)
• use RsyncTask to estimate snapshot size (this is the thing i mentioned in only kill a task if its pid is set #427) This way it can be canceled and run in the background
• made the estimation use the "pulse" progressbar, since it currently doesn't show any real progress anyways