Allowed `dispatch` to take any callables.
Allowed `dispatch` to take any callables.

Review: https://reviews.apache.org/r/46686/

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