SPIKE: Investigate support for partitioning on some table

Some of the database tables on Quay.io are getting absurdly large.

We should find a way to reduce the load that some queries produces when accessing some table.

For example, the ImageStorage, Manifest, ManifestBlob tables could possibly be partitioned by a prefix on the content_digest column (i.e The first few characters of the SHA value), with changes to the application code such that only one partition is accessed at a time.

This would reduce the sheer size of some table and queries/joins associated.

Ref:

- https://architect.readthedocs.io/features/partition/index.html