Prevent multiple instances of working copy pool (#1797)

The working copy pool has to be a singleton, because
otherwise there could be multiple instances with their
own caches and therefore no reuse and maybe more relevant
working directories that will never be deleted.

scm-core/src/main/java/sonia/scm/repository/work/SimpleCachingWorkingCopyPool.java

@@ -34,6 +34,7 @@ import org.slf4j.LoggerFactory;
 import sonia.scm.util.IOUtil;
 
 import javax.inject.Inject;
+import javax.inject.Singleton;
 import java.io.File;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -81,6 +82,7 @@ import static java.util.Optional.of;
  *   <li>Wait for a cached directory on parallel requests</li>
  * </ul>
  */
+@Singleton
 public class SimpleCachingWorkingCopyPool implements WorkingCopyPool {
 
   public static final int DEFAULT_WORKING_COPY_POOL_SIZE = 5;