sql: implicit cross joins can cause OOM errors

[knz]

select4.test from sqllogictest causes my server to be killed by the kernel's OOM killer. It tries a cross join with 6 arguments (FROM a,b,c,d,e,f), and the join first tries to generate millions of cross-product rows in memory.

There are three issues that participate into this problem, which will need separate solutions:

1. the JOIN makes a copy of its right-side data source in memory; if the right-side data source was itself a JOIN, we'll have the copy that JOIN made of its own right operand, plus the copy by the outer JOIN. Ideally, planNode should be extended with a Close() or Purge() interface to release allocated data structures in memory after they are not needed any more.
2. JOINs really ought to be optimized and the filter expressions on the outer SELECT should be pushed down the JOIN operands so that the operands return fewer rows.
3. Really we should not allow queries to allocate all the system's memory.

[knz]

(Found while working on #3292 / #7351)