Clean up row decoding

This patch was authored and released by @gwynne.

Instead of duplicating logic implemented by postgres-kit for row decoding - and incorrectly, at that - we now always call through to that logic. Despite the additional indirection through the SQLRow existential, this should yield a minor improvement in performance, as we perform fewer unnecessary checks and retain fewer copies of data.

Note: Depends on vapor/postgres-kit#221 to receive the full benefit of these changes.