Used 'undiscardable' to protect a future in MesosContainerizer.
Used 'undiscardable' to protect a future in MesosContainerizer.

This patch addressed MESOS-7926 by protecting 'termination.future()'
with 'undiscardable'. This prevented the upstream discard event from
being propagated into 'termination.future()' which gets reused for
other calls.

Review: https://reviews.apache.org/r/62005

verizonlabs/mesos,zmalik/mesos,Gilbert88/mesos,andschwa/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,gsantovena/mesos,abudnik/mesos,verizonlabs/mesos,andschwa/mesos,chhsia0/mesos,kaysoky/mesos,kaysoky/mesos,jpeach/mesos,gsantovena/mesos,zmalik/mesos,verizonlabs/mesos,chhsia0/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,Gilbert88/mesos,andschwa/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,verizonlabs/mesos,dforsyth/mesos,andschwa/mesos,zmalik/mesos,reneploetz/mesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,zmalik/mesos,asamerh4/mesos,abudnik/mesos,abudnik/mesos,asamerh4/mesos,verizonlabs/mesos,dforsyth/mesos,jpeach/mesos,jpeach/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,asamerh4/mesos,asamerh4/mesos,chhsia0/mesos,verizonlabs/mesos,gsantovena/mesos,zmalik/mesos,andschwa/mesos,andschwa/mesos,kaysoky/mesos,dforsyth/mesos,verizonlabs/mesos,Gilbert88/mesos,gsantovena/mesos,kaysoky/mesos,asamerh4/mesos,asamerh4/mesos,asamerh4/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,reneploetz/mesos,chhsia0/mesos,zmalik/mesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos,chhsia0/mesos,gsantovena/mesos,abudnik/mesos,zmalik/mesos,chhsia0/mesos,jpeach/mesos,Gilbert88/mesos