Pull legacy revision query status filters out of the main Query class

Summary:
Ref T2543. Currently, Differential uses a set of hard-coded query filters (like "open" and "closed") to query revisions by status (for example, "open" means any of "review, revision, changes planned, accepted [usually]").

In other applications, like Maniphest, we've replaced this with a low level list of the actual statuses, plus higher level convenience UI through tokenizer functions. This basically has all of the benefits of the hard-coded filters with none of the drawbacks, and is generally more flexible.

I'd like to do that in Differential, too, although we'll need to keep the legacy maps around for a while because they're used by differential.find and differential.getrevision. To prepare for this, pull all the legacy stuff out into a separate class. Then I'll modernize where I can, and we can get rid of this junk some day.

Test Plan: Grepped for RevisionQuery::STATUS. Ran queries via Differential UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18343