FCREPO-1058: DOReaderCache Cleanup

Cleanup of DOReaderCache since it used a pecualiar type unsafe logic to
store cache entries. Changed implentation to use type safe list implementations
and a spring managed thread in order to be JEE compliant

fixed log statements to use slf4j's new {} syntax
Since all cache access must be synch'd, might as well use a TreeMap
bump snapshot version to 3.7.0 for RC