MEDIUM: proxy-proto: don't use buffer flags in conn_si_send_proxy()

MEDIUM: proxy-proto: don't use buffer flags in conn_si_send_proxy()

These ones should only be handled by the stream interface at the end
of the handshake now. Similarly a number of information are now taken
at the connection level rather than at the data level (eg: shutdown).

Fast polling updates have been used instead of slow ones since the
function is only called by the connection handler.