make examples/wsgiserver.py to return the same replies as httpserver.py so that they can be tested with the same test

- do not set SERVER_NAME and SERVER_PORT if they were provided by the user

- store the update environment on 'environ' attribute
- do not set SERVER_NAME and SERVER_PORT if they already were provided by the user
- add get_environ() and use it in the request handler
- add kill() method

server: rewrite StreamServer to use BaseServer and to use a persistent event rather than an accept loop