SQLite Support (#441)

This PR adds support for using SQLite as the system database. This is
intended to be used for lightweight development and testing, allowing
users to get started with DBOS or test their DBOS apps without needing a
Postgres database. It may also be useful for embedded applications.

The SQLite system database is 100% feature complete. To minimize code
duplication, we use SQLAlchemy to implement virtually all system
database operations cross-dialect. The only operations that need custom
implementations in Postgres and SQLite are:

1. Database creation/migration/teardown.
2. Notification management (using LISTEN/NOTIFY in Postgres and polling
in SQLite).
3. Exception categorization (differentiating primary key conflicts from
other errors, for example).

The test matrix has been reconfigured to run all tests against both
Postgres and SQLite. Only a handful of Postgres-specific tests run on
Postgres only.

DBOS now uses SQLite by default if no database URL is provided to it.
This is to make it easier to get started on DBOS—you can now try
it from anywhere without Postgres.