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