AsyncMessenger: fix the leak of file_events

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
AsyncMessenger: reset newly allocated file_events

* otherwise there is chance that Event could take them as already
  initialized ones even they are filled with random bits from heap.
Merge pull request #3255 from tchaikov/async-leak

AsyncMessenger: fix the leak of file_events
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>