colexec: external sort doesn't respect the memory limit well

[yuzefovich]

Currently, the external sort respects the memory limit very poorly - it will hold up to maxNumberPartitions partitions in memory (within each in-memory sorter) each of which is limited by roughly workmem - 1MB, maxNumberPartitions is limited by COCKROACH_VEC_MAX_OPEN_FDS divided by 16 (which equals 256 / 16 = 16 by default), so the external sorter actually might be using on the order of 1 GB of RAM.

In the worst case, however, when the rows are very wide (let's say 1MB in size), then each batch will be on the order of 1GB in size, resulting in the external sorter using something like 16GB of RAM which is likely to lead to OOM crashes. This latter part will be mediated by limiting the batch sizes by workmem setting (#59851).