Fix for UniquePropertyMessageEvictionStrategy memory leak

[Fraser]

Hi all,

I've just submitted a patch for a memory leak I found in the 
implementation of UniquePropertyMessageEvictionStrategy.
The issue and patch can be found here: 
https://issues.apache.org/jira/browse/AMQ-8068 
<https://issues.apache.org/jira/browse/AMQ-8068>

Regards,
Fraser