Add BufferedIterator (#419)

Summary:
This improves performance for datasets that load data lazily. Enabled by default since it shouldn't compromise performance for non-lazy datasets.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/419

Differential Revision: D13546585

Pulled By: myleott

fbshipit-source-id: f6152e2047291b0d68cd7506cd772b0caafe95be