Add an option to switch tokenizers to use "ondemand" instead of "preloaded" datasources

Summary:
The open source Phabricator has like 3,500 user accounts now and it takes a
while to pull/render them. Add an option to switch to ondemand for large
installs.

I'll follow up with a patch at some point to address a couple of name things:

• Denormalize last names into a keyed column (although this evidences some

bias toward the western world).

• Force all usernames to lowercase (sorry Girish, Makinde).

Also this patch is so clean it's crazy.

Didn't bother with other object types for now, I'm planning to dedicate a few
days to Projects at some point and I'll flesh out some auxiliary features like
this when I do that.

Test Plan: Switched to ondemand, verified data was queried dynamically. Switched
back, verified data was preloaded.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, epriestley, nh

Differential Revision: 923