When reloading on dn modification, re-open the connection

This ensures that the content server does not continue to use a deleted
metadata.db.

src/calibre/db/legacy.py

@@ -117,6 +117,7 @@ class LibraryDatabase(object):
 
     def check_if_modified(self):
         if self.last_modified() > self.last_update_check:
+            self.backend.reopen()
             self.new_api.reload_from_db()
         self.last_update_check = utcnow()