MINOR: ssl_sock: make use of CO_FL_WILL_UPDATE

MINOR: ssl_sock: make use of CO_FL_WILL_UPDATE

Now when ssl_sock_{to,from}_buf are called, if the connection doesn't
feature CO_FL_WILL_UPDATE, they will first retrieve the updated flags
using conn_refresh_polling_flags() before changing any flag, then call
conn_cond_update_sock_polling() before leaving, to commit such changes.