Eliminated unnecessary copy in DataEncoder constructor.
Eliminated unnecessary copy in DataEncoder constructor.

With C++11, we can rely on move semantics to save one copy
of the to-be-encoded data.

Additionally this removes an unnecessary forward declaration.

Review: https://reviews.apache.org/r/68061/

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