Core: Update JDBC catalog namespaces

[rdblue]

This fixes a few issues with the JDBC catalog's namespace implementation:

• Uses a default "exists" property to allow creating a database without custom metadata
• Implements loadNamespaceMetadata
• Fixes incorrect NoSuchNamespaceException thrown in dropNamespace (return false instead)
• Fixes dropNamespace when the namespace exists because properties are set (will remove all properties)
• Allows an empty map or set of properties in removeProperties and setProperties as noops
• Moves private helper methods to the end of the JDBCCatalog class
• Update tests for behavior fixes and to use loadNamespaceMetadata rather than directly using the fetchProperties helper method

[danielcweeks]

This may be out of scope for this PR, but these operations don't appear to be atomic.

[danielcweeks]

Might want to create an issue for the setProperties not being atomic, but that is somewhat unrelated to these changes.

[rdblue]

Might want to create an issue for the setProperties not being atomic, but that is somewhat unrelated to these changes.

I think we're about as close as we can get to atomic with generic SQL since there is no standard way to upsert. I've also thought about doing a unique versioned insert and then dropping old versions. That could work to make it slightly more atomic.