Revert "Only create a new model registry if a connection could be established"

This reverts commit 78704e55488073c03b8dbcfb4e605cb003a06a8c.
Revert "Add a registry to each database connection."

This reverts commit 4b11dbe3cf7ff1fec6aeb53599e11ae92930a421.
Make the model registry a singleton.