Rewrite Engine.progress().

Try to make the code more readable and move specific concerns (e.g.,
counting) into own sub-class.

In particular, tasks are now kept in queues so execution order should
be predictable.

gc3libs/__init__.py

@@ -1898,12 +1898,6 @@ def fset(self, value):
                 self.history.append(
                     "Transition from state {0} to state {1}"
                     .format(self._state, value))
-            # update stats on controller -- we need to do this
-            # *after* the `.terminated()` method has gotten a
-            # chance to run and set the final exitcode.
-            if self._ref is not None and self._ref._attached:
-                self._ref._controller._update_task_counts(self._ref, self._state, -1)
-                self._ref._controller._update_task_counts(self._ref, value, +1)
             # finally, update state
             self._state = value