Add LoadTypes method for bulk type registration

[ewhauser]

Is your feature request related to a problem? Please describe.

We are loading a large number of enums/custom domain objects (50+) using AfterConnect and LoadType as described in #1549. The LoadType method issues a SQL query for every type which is inefficient for a large number of enums as it makes a two SQL queries against the pg catalog.

Describe the solution you'd like

A LoadTypes method that takes an slice of strings and returns a slice of pgtype.Type pointers would allow for a single roundtrip to the database.

[ewhauser]

Actually, depending on the type, there are 3-4 queries run per item

[jackc]

Duplicate of #2030.