Initial cred checker.

Summary:
[cred-checker] Initial cred-checker script + refactoring.

[cred-checker] live account table.

[cred-checker] Add some logging support.

[cred-checker] test database.

Refs T139

Test Plan: review code

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T139

Differential Revision: https://review.inboxapp.com/D246