[REVIEW] NXCM-4979: Tasks dissapearing

Related issue
https://issues.sonatype.org/browse/NXCM-4979

This pull does not fix the issue, as it was not reproduced by us. Here are only fixes found during code reading:

* task config manager was leaving configuration "dirty" in some cases
* added a "diagnostic log" that can be enabled on certain systems, that should provide us more information about the main issue. Enabling is possible without putting Nexus into DEBUG mode, to end up with logs filling disk.
* reformat

Related changes in Task Scheduler:
* added logging, to not "swallow" exceptions https://github.com/sonatype/sisu-task-scheduler/commit/a647589427e25d25f9e4fd067456f45c38b4d51a
* variable name cleanup https://github.com/sonatype/sisu-task-scheduler/commit/119dd891bb7d560ee747c8e33fa71066f6f7f3d4
* added "reference" UT https://github.com/sonatype/sisu-task-scheduler/commit/119dd891bb7d560ee747c8e33fa71066f6f7f3d4

As discussed on issue, this change does not make things worse, only improves logging. Hence, is safe to be merged.