add option to handle nil Valuers

[sajanalexander]

This is a fix for #1566 to add an option to handle cases where we've passed a value that is considered nil by anynil, but may have a value which can be substituted, like in the case where something implements the Valuer interface and returns a default value.

Note that I needed to add an option to pgx.Conn to be able to modify the conn.TypeMap(). I couldn't figure out another way to do this when using the stdlib.

Please let me know if this is on the right track and/or needs any modifications.