Changes related to task cancellation

Changes:
* Scheduler will NOT allow update of running but non-cancelable tasks
* proper handling of updates of running and cancellable tasks (cancel, update config, restart)
* EndState and eventing reflecting what tasks "knows" (ie. is unaware of cancelation)
* Future.cancel(true) will now behave as documented: with true will interrupt the thread if task being canceled is not cancelable (but doing that is dangerous)
* logging related improvements (basically https://issues.sonatype.org/browse/NEXUS-7802)

YUM related fixes:
* YUM tasks made cancelable, by working into unique temp folder (to not interfere with each other), and as last step, moving in the data if not canceled

CI
http://bamboo.s/browse/NX-OSSF444
Can `context.getScheduler().getTrigger` ever return `null` or does it always throw `SchedulerException` when it can't find the trigger. (Just trying to work out if the original comment was incorrect, or whether there's a missing check here.)