NEXUS-5749: Scheduled tasks disappears

Multiple problems were probably doing it
* any runtime exception while gathering userId aborts config save
* pool thread got "polluted" by inherited Subjects
* scheduled thread subject was set "too deep", some homework by scheduler
was done outside of it.
* probably NEXUS-5728 caused that inherited subjects become invalid

Issue:
https://issues.sonatype.org/browse/NEXUS-5749

CI:
TBD