Make the storage test fixture connect to postgres itself

Summary:
This avoids reusing a potentially stale connection handle. Also allows testing
potential connection pooling behavior.

This forces us to do proper cursor sanitation as well, a bunch of
"transactional" operations weren't actually transactional.

Test Plan: another round of make test still working

Reviewers: Reviewers!

Differential Revision: https://forge.softwareheritage.org/D323