Skeleton for "Multimeter", a performance sampling application

Summary:
Ref T6930. This application collects and displays performance samples -- roughly, things Phabricator spent some kind of resource on. It will collect samples on different types of resources and events:

• Wall time (queries, service calls, pages)
• Bytes In / Bytes Out (requests)
• Implicit requests to CSS/JS (static resources)

I've started with the simplest case (static resources), since this can be used in an immediate, straghtforward way to improve packaging (look at which individual files have the most requests recently).

There's no aggregation yet and a lot of the data isn't collected properly. Future diffs will add more dimension data (controllers, users), more event and resource types (queries, service calls, wall time), and more display options (aggregation, sorting).

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6930

Differential Revision: https://secure.phabricator.com/D12623