new class_* request for tbl_dbi lazy objects

[alejandrohagan]

I'm not sure if this is an issue but given the growth in database connections and use tbl_dbi objects can we create a new class_* for the lazy tbl objects like "tbl_dbi" ,"tbl_sql" , or "tbl_lazy" ?

[hadley]

This is out of scope for S7, since it's currently focussed on S3 classes provided by base R.