Provide isolated, read/write storage fixtures for unit tests

Summary:

• Unit tests can request storage fixtures.
• We build one fixture across all tests in the process, which can quickstart (takes roughly 1s to build, 200ms to destroy for me). This is a one-time cost for running an arbitrary number of fixture-based tests.
• We isolate all the connections inside transactions for each test, so individual tests don't affect one another.

Test Plan: Ran unit tests, which cover the important properties of fixtures.

Reviewers: btrahan, vrana, jungejason, edward

Reviewed By: btrahan

CC: aran, davidreuss

Maniphest Tasks: T140

Differential Revision: https://secure.phabricator.com/D2345