Simplify FastAbbreviatingPrefixMap to use longestMatch() method of Trie, fix a bug in Trie related to longest matches, enabled abbreviation performance tests since with the Trie improvements in place performance difference is now much more marked

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439954 13f79535-47bb-0310-9956-ffa450edef68