TJ/HM: Reuse fields if you ask for
TJ/HM: Fields are kept in memory for reusing them; So ignore if they exists in tearDown of test
EG/JPM: dropped KeyFilterCache; KeyFilter faster by caching docId to key mapping in array
EG/JPM: introduced KeyValuesCache, made use of in KeyCollector, bypassing CachingKeyCollector, MultiLuceneSingleThreadedTest tests fail
HM/TJ: ignore .classpath etc.