Re: [WIP]Vertical Clustered Index (columnar store extension) - take2

Also, Alvaro seemed to think TAM is the way to go, and in order to keep
the OLTP performance he suggested to use both heap and VCI at the same
time, in different "forks". I'm not sure how would that work, or if we
can already do that - AFAIK we can't, because ForkNumber does not allow
adding custom forks. We'd have to relax that, or invent some sort of
federated TAM (that just multiplexes it to two TAMs). Maybe.

But it's not like the IAM approach doesn't need to do this. The first
patch had to add stuff to a lot of random places to make this work. And
some of the places touch stuff that we don't expect indexes to worry
about, like ALTER TABLE, etc.