HORNETQ-1224 Fix race on QueueImpl.deliver() when paging.
https://bugzilla.redhat.com/show_bug.cgi?id=972444

Case was actually tracked down by Andy Turner and Howard Gao.
QueueImpl, address synchronization issues:
1. synchronization of 'paused' was inconsistent.
2. messagesAdded is only accessed inside synchronized blocks, so
   it need not be volatile
Merge pull request #1119 from FranciscoBorges/master

HORNETQ-1224 Fix race on QueueImpl.deliver() when paging. https://bugzilla.redhat.com/show_bug.cgi?id=972444