Moved around the `Request` struct in the file.
Moved around the `Request` struct in the file.

This serves two purposes:
- This brings it near the `Response` struct thereby making it easy
to switch between them for readability.
- In the future, `Request` would have a `Pipe::Reader` member
for supporting request streaming. Otherwise, we would need to
add another forward declaration and forward declaring nested
structs is not quite possible.

Review: https://reviews.apache.org/r/53481/

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