Bugzilla #46962: Fixed deadlock in PropertyCache.

Bugzilla #46962: Fixed deadlock in PropertyCache.

Re-wrote the PropertyCache class, leveraging Java 1.5 concurrency features.
Implemented equals and hashCode on many Property sub-classes to make the caching more effective

Patch by Alexios Giotis, applied with the following minor modifications:
* Javadoc improvements to PropertyCache
* Factorize into new CompareUtil class code often used by equals and hashCode implementations

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@1303891 13f79535-47bb-0310-9956-ffa450edef68