https://issues.apache.org/jira/browse/AMQ-4249 - Simplified the race
condition-fix.

- The read-write lock should not be required since
PrincipalProperties is immutable once constructed.
- Removed the unneccessary copy in the group traversal, iterating over
the Properties-entries seems to be threadsafe already.
- Added pool shutdown and cache-cleanup to ensure that the race
condition test runs in isolation without influencing later tests.