colexec: tune reserved memory for partitions

[asubiotto]

Sorts:

• We currently reserve half of the work memory in sorts. With a partition cache size of 64KiB and a default work mem of 64MiB, this allows us to have 1024 partitions open at any one time. This seems like too many partitions open at one time and reduces the partition size to 32MiB. We might be able to find a better balance.
  Joins:
• We limit ourselves to 16 partitions in this case. A lower number of partitions increases the chances that the partitions are large and the chances that we'll have to repartition. At least it should be fair and the number of partitions should be equal to sorts.

To find a good number of partitions to set the defaults to, it would be good to run end-to-end performance benchmarking on tpch.

Jira issue: CRDB-5143

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