Add `bin/worker flood`, for flooding the task queue with work

Summary: Ref T6615. Ref T3554. We need better tooling around the queue eventually, so start here.

Test Plan: Added 100K+ tasks locally with `bin/worker flood`. Executed some of them with `bin/phd debug taskmaster` (we already have a TestWorker, used in unit tests).

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3554, T6615

Differential Revision: https://secure.phabricator.com/D10894