SQLite storage back-end for self-contained deployment

[dylan-bourque]

The initial implementation for Perseus uses a PostgreSQL database as the storage layer. This well-worn path is simple and functional, but does require deploying and maintaining a separate PG instance. There is also the gotcha that Amazon RDS doesn't support the pg-semver PG extension that we use for storing semver values.

One alternative that would simplify things a bit would be to add support for an embedded SQLite database as an alternative to PostgreSQL. This would be ideal for a smaller "customer" that wants to run Perseus on a single, self-contained node and won't have too big of a storage load.

This seems like a valid option, with the caveat that it may not be easy to implement the full SemVer specification within the database like we have with the pg-semver PG extension.