RWLock in queue does not used properly - performance improvement

The full story is in the user forum under title:
Activemq 5.4.2 hangs when the temp disk usage is used

The advantage of RW locks in Queue class is not used much. The cursors objects are synchronized, so the readers cannot access the queued messages concurrently.

The fixing effort of this issue may not be worth it, but you may consider this issue during fixing related issues.