AsyncMessenger: should retry in case of EINTR

* if EINTR is returned, do_send() should return 0 or `continue`.
* reword the comment a little bit

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
AsyncMessenger: s/sended/sent/

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
AsyncMessender: use IOV_MAX instead of IOV_LEN

Signed-off-by: Kefu Chai <tchaikov@gmail.com>