Commit to PostgreSQL

[jace]

Coaster currently provides much optional functionality that depends on PostgreSQL, providing workarounds for other databases. The JsonDict type in coaster.sqlalchemy, for instance, does client-side JSON parsing if the server isn't PostgreSQL.

However, since almost all our apps now depend on PostgreSQL and will not run on another database, the workarounds in Coaster are accumulated cruft preventing further commitment to PostgreSQL features such as xmin-based versioning.

Should Coaster depend on PostgreSQL as baseline, dropping support for other databases?

[jace]

The xmin example here can be resolved with an optional mixin class that client apps can use. If it turns out to be harmless on SQLite and our current use cases, we add it to the base classes.

SQLite support is useful for tests, so we shouldn't abandon it entirely.