Fixes #95

Fixes #96

IP addresses are now always stored in v6 format. The original tests
supported the incorrect assumption that IPs were mapped as versioned, so
fixing this caused many of them to break. This patch thus also fixes all
the tests that failed to support v6 mapping.

Also sets used_by_tenant_id on IP addresses correctly after a migration
months ago changed tenant_id -> used_by_tenant_id. Apparently,
sqlalchemy ignores any unmapped attributes. Finally, adds a test so it
won't happen again.