Single-use, non-recursive common table expressions (CTEs) - WITH clause

[jseldess]

Background: https://airtable.com/tblD3oZPLJgGhCmch/viwINAgXA5on9pr2T/reclGbRZtI6TME4JT
RFC: https://github.com/cockroachdb/cockroach/blob/master/docs/RFCS/20171206_single_use_common_table_expressions.md

PRs:

• sql: single-use common table expression support

[knz]

If that's OK, I'd like to take this over and complement #2762 with this new feature. Does this work for you?

[lnhsingh]

@knz - that's fine with me, as long as @jseldess is ok with it. i haven't started writing anything yet, but have been getting familiar with the topic so i can be the reviewer on this. @jseldess, is that ok? if so, i can take over a different topic.

[jseldess]

Fine with me, too. Thanks, @knz!

@lhirata, let's sync on another 2.0 project you can help with.