Clear all callbacks when a future is completed.
Clear all callbacks when a future is completed.

Review: https://reviews.apache.org/r/30348

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