WebSession: new pure mod_python based sessions

• New session handling based on pure mod_python session implementation. (Fixes some issues when running under Python 2.6.)

• Note: the _enhance_req() trick is required to support very old mod_python versions. It may be better run as the first instruction in the _profiler() function body already. But since we are going to drop support for very old mod_python versions soon (see next commit), this subtle placement difference is not so important here.