handle multiple pool keys

[mccanne]

e.g., primary, second etc keys should be allowed for pools. The data structures are all set up for this but the compiler and optimizer currently assume a single primary key.

Also, dag.Merge will need to handle multiple keys and dag.Merge.Reverse + Key should be replaced with order.Layout.

[philrz]

@mccanne senses he may be able to knock this one out quickly shortly after he addresses #2482, but if this turns out to be harder than we thought, we agreed that we can push it out to MVP1.

[philrz]

Note to self: When this gets implemented, remove the reference to this issue from the Zed lake design doc.

[mccanne]

optimizer needs to handle multi-sort keys too

[philrz]

We discussed this one again recently and figured we could continue postponing implementation until we have more traction with the lake. But in the short term we should have the tooling prevent the configuration of multiple pool keys and kick back a message referencing this issue so we can find out if there's demand from users that need it. (#4596)