Deprecate org.infinispan.commons.util.Base64 and replace usages with java.util.Base64

See http://java-performance.info/base64-encoding-and-decoding-performance/ for a discussion on performance of various implementations of Base64 encoding.
The java 8 implementation is slightly faster than the 'Robert Harder' implementation we have in org.infinispan.commons.util.Base64 class.