Used variadic templates for protobuf handlers.
Used variadic templates for protobuf handlers.

This simplifies code of `ProtobufProcess` by replacing it with variadic
templates implementation.

Review: https://reviews.apache.org/r/62898/

kaysoky/mesos,jpeach/mesos,chhsia0/mesos,asamerh4/mesos,reneploetz/mesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,Gilbert88/mesos,jpeach/mesos,abudnik/mesos,zmalik/mesos,zmalik/mesos,kaysoky/mesos,gsantovena/mesos,zmalik/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,Gilbert88/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,kaysoky/mesos,jpeach/mesos,abudnik/mesos,asamerh4/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,asamerh4/mesos,reneploetz/mesos,Gilbert88/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,andschwa/mesos,abudnik/mesos,andschwa/mesos,gsantovena/mesos,chhsia0/mesos,zmalik/mesos,Gilbert88/mesos,jpeach/mesos,asamerh4/mesos,gsantovena/mesos,asamerh4/mesos,abudnik/mesos,chhsia0/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,zmalik/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,abudnik/mesos,reneploetz/mesos,zmalik/mesos,dforsyth/mesos,chhsia0/mesos,asamerh4/mesos,chhsia0/mesos,zmalik/mesos,dforsyth/mesos,andschwa/mesos,andschwa/mesos,gsantovena/mesos,Gilbert88/mesos,gsantovena/mesos,asamerh4/mesos