NEXUS-7789: Fixing IT and fixing bug it revealed

After fixing the ignored Nexus4066TaskMutualExclusionIT it
revealed that mutual exclusion is "less strict" that it should,
it does not consider transitive group members.

IT fixed and enabled, and bug fixed.

Issue
https://issues.sonatype.org/browse/NEXUS-7789

CI
http://bamboo.s/browse/NX-OSSF437
FYI `Collections.disjoint` can be faster as it doesn't create a new collection/view, and optimizes for a couple of cases which guava doesn't (such as preferring to iterate over the smaller collection).