Add new mempool benchmarks for a complex pool #17292

This PR is related to #17268.

It adds a mempool stress test which makes a really big complicated tx graph, and then, similar to mempool_eviction test, trims the size.

The test setup is to make 100 original transactions with Rand(10)+2 outputs each.

Then, 800 times:

we create a new transaction with Rand(10) + 1 parents that are randomly sampled from all existing transactions (with unspent outputs). From each such parent, we then select Rand(remaining outputs) +1 50% of the time, or 1 outputs 50% of the time.

Then, we trim the size to 3/4. Then we trim it to just a single transaction.

This creates, hopefully, a big bundle of transactions with lots of complex structure, that should really put a strain on the mempool graph algorithms.

This ends up testing both the descendant and ancestor tracking.

I don't love that the test is "unstable". That is, in order to compare this test to another, you really can't modify any of the internal state because it will have a different order of invocations of the deterministic randomness. However, it certainly suffices for comparing branches.

For those curious, this is a graph of the transactions that are being tested here. The red ones are the "parentless" ones that we create (along with the fake parent I put in to make the graph render nicely.

This graph doesn't show the weights for how many outputs are being consumed by a child from a parent, an inbound edge simply indicates any amount of consumption.

Concept ACK! I was waiting for this

concept ACK! moar data

Moar data:

I ran #17268 against master across a number of paramters for the number of transactions.

What's interesting about the testing setup is if the only modification you make is the number of transactions, it is "monotonic" that is, G(n) is a subgraph of G(n+1) for all n. Adjusting other parameters will not have the same sort of effect, unfortunately, as the construction is not stable.

Concept ACK

Not as useful to look at, but this is the structure with 10,000 nodes and 800. If anyone wants the svg (fully zoomable), can share.

This is made using the twopi shell like layout. The other layout engines choke with this many nodes.