Updated balloon executor.
Updated balloon executor.

Fixes the logic in the example `balloon-executor` to work in cases not
exercised by the `ROOT_CGROUPS_BalloonFramework` test.

* The "task" logic was moved into a separate thread.  This fixes the
  case where the balloon executor does not exceed the memory limit.
  (i.e. by Unblocking the driver's thread while sending status updates.)
* Changed log messages to use glog.
* Added logic to prevent multiple task launches with the same executor.

Review: https://reviews.apache.org/r/46407/

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