Fixed IOSwitchboard waiting EOF from attach container input request.
Fixed IOSwitchboard waiting EOF from attach container input request.

Previously, when a corresponding nested container terminated, while the
user was attached to the container's stdin via `ATTACH_CONTAINER_INPUT`
IOSwitchboard didn't terminate immediately. IOSwitchboard was waiting
for EOF message from the input HTTP connection. Since the IOSwitchboard
was stuck, the corresponding nested container was also stuck in
`DESTROYING` state.

This patch fixes the aforementioned issue by sending 200 `OK` response
for `ATTACH_CONTAINER_INPUT` call in the case when io redirect is
finished while reading from the HTTP input connection is not.

Review: https://reviews.apache.org/r/68232/

reneploetz/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,jpeach/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,Gilbert88/mesos,gsantovena/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,jpeach/mesos,dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,gsantovena/mesos,gsantovena/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,jpeach/mesos