Ensured default executor ignores health updates for terminated tasks.
Ensured default executor ignores health updates for terminated tasks.

After the task has been terminated, its health updates become
irrelevant and should be ignored. Also if the default executor
shuts down, we can safely stop all health checkers.

Technically health checking should be stopped right before
TASK_KILLING update is sent to avoid subsequent TASK_RUNNING
updates, but the default executor currently does not support
TASK_KILLING.

Review: https://reviews.apache.org/r/52871/

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