Add a RefPosition table to hold branch/tag positions once the RefCursor table is split

Summary:
Ref T11823. Currently, we have a "RefCursor" table which stores rows like <branch or tag name, commit it is pointing at> with some more data.

Because Mercurial can have a single branch pointing at several different places, this table must allow multiple rows with the same branch or tag name.

Among other things, this means there isn't a single PHID which can be used to identify a branch name in a stable way. However, we have several UIs where we want to be able to do this.

Some specific examples where we run into trouble: in Mercurial, if there are 5 heads for "default", that means there are 5 phids. And currently, if someone deletes a branch, we lose the PHID for it. Instead, we'd rather retain it so the whole world doesn't break if you accidentally delete a branch and then fix it a little later.

(I'll likely hold this until the rest of the logic is fleshed out a little more in followup changes.)

Test Plan: Ran bin/storage upgrade, saw the table get created without warnings.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11823

Differential Revision: https://secure.phabricator.com/D18602