IndexReader.isDeleted synchronization

[Jason Rutherglen]

A possible solution to the IndexReader.isDeleted synchronization is to have
a delegate class that is different depending on the Java version.  For Java
1.4 the class defaults to the synchronization used now.  A Java 1.5 version
would use a volatile deletedDocs attribute.  This would provide a backwards
compatible solution and a solution for Java versions with volatile working
that can avoid the use of synchronized.