[C++][DISCUSS] Sequencing in ExecPlan

[Weston Pace <we...@gmail.com>]

The topic of sequencing to achieve things like deterministic output of
head/tail type operations has come up a few times.  I went ahead and
drew up a proposal for adding these concepts into the exec plan via a
batch index.  It was just going to be an email but it got a bit long
so I put it in a Google doc.  I've heard different opinions on the
topic so I thought it might be good to gather some consensus / make
sure I'm not crazy before embarking on any actual work.

Also, I'm not sure what DataFusion has in place, so I'd welcome any
thoughts there too.

Proposal: https://docs.google.com/document/d/1MfVE9td9D4n5y-PTn66kk4-9xG7feXs1zSFf-qxQgPs/edit?usp=sharing