Add explicit e2e load test

[evan-forbes]

We have manual load tests elsewhere, however we should add an explicit automated e2e load test where we have a high enough load in the mempool that we expect full blocks.

This would involve starting ~4 knuu nodes and an equivalent number of txsim instances, enough txsim load to fill blocks some multiple of the capacity (8 is fine imo for example).

bonus: Ideally, we'd also test that the txs are included within a specific period of time.

[evan-forbes]

the load test can analyze block times using basic stats

extracting a gist here from #3882 that @staheri14 wrote https://gist.github.com/evan-forbes/3674902cffbdbc06e85ddf763f381f2f

[staheri14]

The e2e benhcamrk tests do the load test already, I think the only thing to do is to add it to the CI.