Support timestamptz type

[JelteF]

Generally it's recommended to use the timestamptz type instead of the timestamp type, so we should support this. I think supporting this is probably very simple as it's stored the same on disk as the timestamp type, but it needs a few tests too. If easy, it would be good to support filtering on timestamptz too.

[wuputah]

This type is very commonly used and to me this seems like an oversight for 0.1; if this is just a few lines of code it would be great to get this done.