release-20.1: rowexec: release buckets from hash aggregator eagerly

[yuzefovich]

Backport 1/2 commits from #47466.

/cc @cockroachdb/release

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rowexec: release buckets from hash aggregator eagerly

This commit makes hash aggregator release the memory under buckets
eagerly (once we're done with the bucket) so that it is returned to the
system. This can matter a lot when we have large number of buckets (on
the order of 100k). Previously, this would happen only on the flow
shutdown, once we're losing the references to hashAggregator
processor. But it was problematic - we "released" the associated memory
from the memory accounting, yet we were holding the references still.
With this commit we will reduce the memory footprint and we'll be a lot
closer to what our memory accounting thinks we're using.

Fixes: #47205.

Release note (bug fix): Previously, CockroachDB was incorrectly
releasing memory used by hash aggregation (we were releasing the correct
amount from the internal memory accounting system but, by mistake, were
keeping the references to the actual memory for some time which
prohibited the memory to be garbage collected). This could lead to
a crash (which was more likely when hash aggregation had store on the
order of 100k of groups) and is now fixed.

[cockroach-teamcity]

This change is 

[yuzefovich]

I think I would call it a soft release blocker and would like to include it in 20.1.0 release.

[asubiotto]

I think this is probably going to have to wait until 20.1.0 is out the door. This issue does cause a server crash, but only in cases where there is an aggregation with a very large number of groups (seen with 500k distinct groups). I think this is enough of an edge case that we want to hold off since I think we're at the point where we're only merging stuff we would consider delaying the release for.

[yuzefovich]

SGTM.