Added change of fields `Request` to optional to CHANGELOG.
Added change of fields `Request` to optional to CHANGELOG.

The changed semantic of the `Request` protobug message with MESOS-5405
should be added to the CHANGELOG.

Review: https://reviews.apache.org/r/48267/

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