replace ludb with data-store

LuDB has a few disadvantages. 
  1. Most notably disk space. H2 wastes a lot of valuable disk space.
     For my test data set with 44 million entries it is 14 MB 
     (sometimes a lot more; depends on H2 internal cleanup). With 
     data-store it is 15 KB.
     Overall I could reduce the disk space from 231 MB to 200 MB (13.4 %
     in this example). That is an average of 4.6 bytes per entry.
  2. Speed:
     a) Liquibase is slow. The first time it takes approx. three seconds
     b) Query and insertion. with data-store we can insert entries 
        up to 1.6 times faster.

Data-store uses a few tricks to save disk space:
  1. We encode the tags into the file names.
  2. To keep them short we translate the key/value of the tag into 
     shorter numbers. For example "foo" -> 12 and "bar" to 47. So the
     tag "foo"/"bar" would be 12/47. 
     We then translate this number into a numeral system of base 62
     (a-zA-Z0-9), so it can be used for file names and it is shorter.
     That way we only have to store the mapping of string to int.
  3. We do that in a simple tab separated file.

file-utils/src/main/java/org/lucares/utils/file/FileUtils.java

@@ -0,0 +1,69 @@
+package org.lucares.utils.file;
+
+import java.io.IOException;
+import java.nio.file.FileVisitResult;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.SimpleFileVisitor;
+import java.nio.file.attribute.BasicFileAttributes;
+import java.util.List;
+import java.util.function.BiPredicate;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class FileUtils {
+	private static final Logger LOGGER = LoggerFactory.getLogger(FileUtils.class);
+
+	private static final class RecursiveDeleter extends SimpleFileVisitor<Path> {
+
+		@Override
+		public FileVisitResult visitFile(final Path file, final BasicFileAttributes attrs) throws IOException {
+
+			Files.delete(file);
+			LOGGER.trace("deleted: {}", file);
+
+			return FileVisitResult.CONTINUE;
+		}
+
+		@Override
+		public FileVisitResult postVisitDirectory(final Path dir, final IOException exc) throws IOException {
+
+			Files.delete(dir);
+			LOGGER.trace("deleted: {}", dir);
+
+			return FileVisitResult.CONTINUE;
+		}
+	}
+
+	public static void delete(final Path path) {
+
+		final int maxAttempts = 10;
+		int attempt = 1;
+
+		while (attempt <= maxAttempts) {
+			try {
+				LOGGER.debug("deleting '{}' attempt {} of {}", path.toFile().getAbsolutePath(), attempt, maxAttempts);
+				Files.walkFileTree(path, new RecursiveDeleter());
+				break;
+			} catch (final IOException e) {
+				final String msg = "failed to delete '" + path.toFile().getAbsolutePath() + "' on attempt " + attempt
+						+ " of " + maxAttempts;
+				LOGGER.warn(msg, e);
+			}
+			attempt++;
+		}
+	}
+
+	public static List<Path> listRecursively(final Path start) throws IOException {
+
+		final int maxDepth = Integer.MAX_VALUE;
+		final BiPredicate<Path, BasicFileAttributes> matcher = (path, attr) -> Files.isRegularFile(path);
+
+		try (final Stream<Path> files = Files.find(start, maxDepth, matcher)) {
+			return files.collect(Collectors.toList());
+		}
+	}
+}