core: make http_request reply with 500 in __dealloc__ if no reply was sent; make http class responsible for detaching http_requests on connection close;

- make http_request weak-referencable
- remove set_closecb() method from http_connection (it is used internally now, to detach http_requests)
- remove set_gencb() from http class; pass the callback to the constructor instead
- remove set_cb() from http class: it does not fit very well with the current design and is not used by gevent itself; use issue tracker if you need this back

core: when reporting 500 Internal Error, add "Connection: close" and "Content-type: text/plain" headers

socket: remove deprecated functions: socket_bind_and_listen, set_reuse_addr, connect_tcp and tcp_server

socket: fix bug in dup() - it did not inherit 'timeout' of the original socket; bug introduced in 0.12.0