Made the default executor's handling of kill escalations more robust.
Made the default executor's handling of kill escalations more robust.

This patch makes the default executor retry SIGKILL escalations if the
executor is disconnected from the agent or the kill call fails.

Review: https://reviews.apache.org/r/65694/

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