Update table schema for "AffectedPath" table

Summary:
Ref T13639. Make schema changes:

• Make repositoryID nullable, for revisions with no repository.
• Remove "epoch", which has no readers and no clear use.
• Change the ordering of the key, since "pathID" has more unique values and no queries ever issue without it.

Test Plan:

• Ran bin/storage upgrade, got a clean schema.
• Reindexed all revisions with an external script.
• Reviewed index via debug UI, saw appropriate index for non-repositoy revisions.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13639

Differential Revision: https://secure.phabricator.com/D21617