Purge the query cache to prevent GC livelocks

[teh-cmc]

When the GC needs to reclaim memory but has already exhausted all the data from the store, then purge the query cache directly.
The complete rationale for this patch is there: #7369 (comment).

Before:

After:

• Fixes GC livelocks when temporal data is exhausted #7369

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[ricpruss]

Sorry this is not yet fixed in the latest nightly build for us.

I can send another recording if you need. Possibly something else is breaking this again.

[jleibs]

It looks like our nightly might have been red for a while in there -- just to double-check, could you confirm the sha in the about panel of the viewer where you're seeing the issue?

e.g.

[ricpruss]

Good news! We had rerun going at full power for 30 mins with a 4GB limit enabled. It hasn't really gone over 3.3GB. We super happy thanks!