Limit maximum size of Owners package queries

Summary:
Currently, a change may affect a very large number of paths. When we run the OwnersWorker on it, we'll execute a query which looks up packages for the paths. This may exceed "max_allowed_packet". Instead, break the list of paths into smaller chunks.

This is mostly to unblock r4nt / llvm, I'm going to add a more finessed approach to array_chunk() shortly.

Test Plan: Ran reparse.php --owners on a revision which affected packages, verified results are the same before and after the change. Set chunk size to 1, verified query results aggregated properly.

Reviewers: btrahan, jungejason, nh

Reviewed By: jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2943