pool: rename GreenletSet to Group. The old name is available as an alias (will be removed in the future)

also use a message that is a bit different from standard in this case

core: make sure we never incref events more than once and never decref them more than we increfed them. Thanks to Cyril Bay.

This fixes Cyril's test as well as test__server_pywsgi.py/test__server.TestDefaultSpawn.test_error_in_spawn which DECREFed event twice (detected by python-dbg).

socket: make recv(), send() and other methods keep the reference to socket so that fd is not closed while we're waiting

- also expect waitxxx() to be cancelled and exit appropriatelly in such case

socket: if socket closed during connect() in another greenlet, make connect() raise socket.error(EBADF)

socket: if socket closed during accept() call in another greenlet, make accept() raise socket.error(EBADF)

socket: make wait_read/write/readwrite functions return True if they return normally and False if they were cancelled with cancel_wait()

add server_implements_pipeline option to test__pywsgi.py which disables tests that depend on pipelining working in test__wsgi.py

- core.http: remove unused '_gencb' property
- core.http: make 'handle' property public
- core.http: add default_response_headers argument and property that is passed to each http_request

the old way of settings _default_response_code is no longer supported by core