Update app to be more idiomatic, follow CRDB best practices

[ericharmeling]

We really want all sample apps for fully-supported tools to be idiomatic to their language/framework, and to follow fundamental CockroachDB best practices.

We recommend updating the sample app to follow this generalized sample application specification. This app specification should be used as a high-level template that is compatible with language/tooling conventions. Note that the spec actually calls for two apps:

1. A simplified, true "Hello World" app that just connects to the DB.
2. A simple CRUD app that implements pretty much the same functionality as the current sample app.

We've already updated the sample apps for some tools to do this:

• Go/pgx: hello-world-go-pgx and example-app-go-pgx
• Go/GORM: hello-world-go-gorm and example-app-go-gorm
• Python/SQLAlchemy: hello-world-python-sqlalchemy and example-app-python-sqlalchemy
• node.JS/node-postgres: hello-world-node-postgres and example-app-node-postgres
• Java/JDBC: hello-world-java-jdbc and example-app-java-jdbc