Add time measurement per task.
Use time of -1 if the task was not evaluated last time.
Add logging of number of evaluations.
Auto unboxing does not work the way I expected.
Persist time and evaluations.
Measure using ThreadMXBean.
Merge branch 'feature/measure-task-time'

Conflicts:
	org.metaborg.runtime.task/src/main/java/org/metaborg/runtime/task/TaskEvaluator.java