Add JSON type OID to type cache

This is so that it won't be looked up when in one of the JSON output formats, because it won't then be able to successfully parse the results of the INFORMATION_SCHEMA queries if we make that the type used.

The alternative fix would be to make those queries immune to the output format setting. For example, by adding some new keyword to the SELECT statement. I'm certainly not opposed to that approach, but it is more machinery throughout the stack.