Improve .cpuprofile download

Summary:
- Include request timestamp in filename to make finding the correct file easier
- Use URL field properly instead of including in "function name"
- Set callUID to a unique per-function ID instead of the call ID to match Chrome's behavior, allowing calls to the same function to be grouped together properly

Test Plan:
Profiled loading /style with the old and new code.

Old:

{F51040}

New:

{F51042}

(Not exactly the same profile, but note that readline is now properly one entry instead of 3+ in the first screenshot.)

Reviewers: chris, alan

Reviewed By: alan

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D12799