bug: mempool block-building integration appears to be inefficient, leading to building small blocks

[l-monninger]

Summary

The current manner of building blocks from the mempool seems to consistently blocks with a single-digit number of transcations under high load. This is problematic because it makes it less likely that we can take advantage of Block-STM.

Steps to Reproduce

Similar to #110 this can be observed on the dev branch. This issues are actually likely very much related in this particular reproduction because of the problem with account creation.