Added healthy field to TaskUpdate.
Added healthy field to TaskUpdate.

As a part of the health check story, this patch adds an optional
healthy field to the task update proto.  The semanticist should be
understood as, if the field is absent - no health check has (yet) been
carried out.  If present, it will either encode unhealthy
(healthy=false) or healthy determined by the HealthCheck protobuf in
the command info proto.  This means that multiple TASK_RUNNING updates
can be sent for a task, updating the state of the last health check.

Review: https://reviews.apache.org/r/21499

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