MDL-41017 improve purify caching

The improvements include:

* HTMLPurifier cache is stored in localcachedir
* allowobjectembed changes are not ignored any more
* the cache keys include revision and all options which makes
  this suitable for local caches on cluster nodes
* unchanged test is replaced by "true" value which should
  significantly improve performance
* removal of purge_all_caches() hack for directory recreation
* comments and coding style cleanup

lib/testing/classes/util.php

@@ -598,7 +598,6 @@ abstract class testing_util {
         closedir($handle);
         make_temp_directory('');
         make_cache_directory('');
-        make_cache_directory('htmlpurifier');
         make_localcache_directory('');
         // Reset the cache API so that it recreates it's required directories as well.
         cache_factory::reset();