LUCENE-5887: Remove WeakIdentityMap caching in AttributeFactory, AttributeSource, and VirtualMethod in favour of Java 7's ClassValue. Always use MethodHandles to create AttributeImpl classes.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1618148 13f79535-47bb-0310-9956-ffa450edef68