R1:6514237c0e01

Implement an iterator for build log chunks

Summary: Ref T5822. This will make it easier to compress and archive chunks without needing to hold them in memory.

Test Plan: Ran a build, looked at some logs.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5822

Differential Revision: https://secure.phabricator.com/D15378