fixes to usage of no-cache: use and recognize also the no-store
directive
fixes for searches when initialization of large autotagging libraries
have not been finished
preventing the use of no-cache and expires in case that images are
generated dynamically which will stay static in the future. This applies
mainly to the search result favicon in front of search hits. These icons
will now be generated once, but then caches in the browser. There is
also a YaCy-internal cache for these icons which had prevented the
re-generation of the icons in YaCy, but this cache is now superfluous
since the browser should not call the servlet ViewImage again.