changed table file extension to .ldb

the latest c++ leveldb uses the file extension .ldb for its table files (for some windows compatibility issue). I've switched the filenames to also use the .ldb extension to resolve compatibility. The .sst extension will still be recognized in opening files. Also changed a pom to include the latest jni-leveldb (which uses a more recent c++ backend) so that interop tests with the new extension will work