Investigate increased memory usage while syncing with Genesis in 10.5

@karknu has performed some experiments with full syncs using 10.5 (which is not yet released, but using `ouroboros-consensus-0.27` which is intended for 10.5, with some additional patches), and reports an increase in memory, causing the node to crash with a 28GB heap limit:

> The memory increase only happens with genesis enabled but I can’t tell if it is a space leak or genesis just requires slightly more memory (more active peers => more memory)

The goal of this ticket is to find out whether there is a new serious memory leak (which we must fix) or whether the memory requirement just grew slightly (eg it is expected that #1288 should increase peak memory usage *slightly*), which is not a big problem.

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We know that 10.4.1 definitely doesn't have a (serious) leak; Nick did a full sync, and the [SDET sync tests](https://tests.cardano.intersectmbo.org/test_results/sync_tests.html) also support this. We do know that Genesis uses a bit more memory than Praos (goes away after a restart once caught-up), which would be nice to fully understand, but it is low priority.

Reproduce a full sync with the 10.5 setup using ouroboros-consensus-0.27, comparing Genesis with Praos and the known 10.4.1 behavior. Review the SDET sync test results and measure whether memory continues growing or settles after catch-up. Done means determining whether this is a serious leak or an expected increase in peak memory, with evidence documented.

Written by the indexing model from the issue text.