Previously we would merge all reports in a 15-second window.
Now we use a 'quantum' of 3 seconds, similar to the single-user app.
E.g. a 30-node cluster will have 150 individual reports over 15
seconds, but the new code will merge 5 pre-merged reports plus 20-ish
very recent individual ones.
This limits the max heap size used for deserialising, since we only do
3 seconds at once per instance.
Individual reports are still put into the cache, but should get
displaced by the pre-merged ones under LRU.