Clear manifest list cache before pruning.

The manifest list cache *should* be updated timely and correctly now
after recent changes. Since pruning is not performance critical, we
avoid any issues right away beforehand just to be sure by clearing it.

Bug: WPL-2473
Change-Id: Ieebc4725d78f945a3edd996a3b9b8e6a8c8ba512

bhive/src/main/java/io/bdeploy/bhive/op/PruneOperation.java

@@ -52,6 +52,9 @@ public SortedMap<ObjectId, Long> call() throws Exception {
             //  3) Upon completion, existing trasactions will block removal of the markers until the root is unlocked.
             execute(new LockDirectoryOperation().setDirectory(getMarkerRoot()));
 
+            // make sure there is no outdated information
+            getManifestDatabase().invalidateCaches();
+
             SortedSet<ObjectId> all;
             try {
                 // read existing manifests also inside the lock, so we are sure that the existing