Fixed a use-after-close bug in PollSocketImpl.
Fixed a use-after-close bug in PollSocketImpl.

Currently, the poll socket implementation doesn't keep
the socket alive after `PollSocketImpl.send(data, length)`
passes execution off to a continuation and so it may send
on a stale file descriptor!

Review: https://reviews.apache.org/r/50936/

jpeach/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,verizonlabs/mesos,neilconway/mesos,neilconway/mesos,chhsia0/mesos,andschwa/mesos,jpeach/mesos,craimbert/mesos,craimbert/mesos,zmalik/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,gsantovena/mesos,verizonlabs/mesos,gsantovena/mesos,jpeach/mesos,zmalik/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,abudnik/mesos,chhsia0/mesos,yuquanshan/customizedMesos,jpeach/mesos,zmalik/mesos,gsantovena/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,Gilbert88/mesos,reneploetz/mesos,craimbert/mesos,dforsyth/mesos,chhsia0/mesos,kaysoky/mesos,verizonlabs/mesos,reneploetz/mesos,asamerh4/mesos,reneploetz/mesos,gsantovena/mesos,neilconway/mesos,neilconway/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,gsantovena/mesos,yuquanshan/customizedMesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,craimbert/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,Gilbert88/mesos,craimbert/mesos,asamerh4/mesos,zmalik/mesos,jpeach/mesos,asamerh4/mesos,asamerh4/mesos,reneploetz/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,abudnik/mesos,dforsyth/mesos,verizonlabs/mesos,andschwa/mesos,jpeach/mesos,verizonlabs/mesos,asamerh4/mesos,abudnik/mesos,dforsyth/mesos,chhsia0/mesos,zmalik/mesos,andschwa/mesos,neilconway/mesos,yuquanshan/customizedMesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,neilconway/mesos,chhsia0/mesos,reneploetz/mesos,asamerh4/mesos,asamerh4/mesos,andschwa/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,verizonlabs/mesos,andschwa/mesos,abudnik/mesos,Gilbert88/mesos,craimbert/mesos,neilconway/mesos,gsantovena/mesos,dforsyth/mesos,verizonlabs/mesos