Move trash folders deletion in background

Deleting trash folders can take some time so execute in a separate
thread to not slow down Gerrit start up.

Change-Id: I07daa18ebcad41538186f97550a7f85faa074b4d
diff --git a/src/main/java/com/googlesource/gerrit/plugins/deleteproject/fs/DeleteTrashFolders.java b/src/main/java/com/googlesource/gerrit/plugins/deleteproject/fs/DeleteTrashFolders.java
index 7e6b4aa..b39b371 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/deleteproject/fs/DeleteTrashFolders.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/deleteproject/fs/DeleteTrashFolders.java
@@ -94,11 +94,16 @@
 
   @Override
   public void start() {
-    try {
-      Files.walkFileTree(gitDir.toPath(), new TrashFolderRemover());
-    } catch (IOException e) {
-      log.warn("Exception occured while trying to delete trash folders", e);
-    }
+    new Thread(new Runnable() {
+      @Override
+      public void run() {
+        try {
+          Files.walkFileTree(gitDir.toPath(), new TrashFolderRemover());
+        } catch (IOException e) {
+          log.warn("Exception occured while trying to delete trash folders", e);
+        }
+      }
+    }, "DeleteTrashFolders").start();
   }
 
   @Override