Transaction selection

[mathcrypto]

spacemeshos/SMIPS#50

https://community.spacemesh.io/t/selection-algorithm-for-the-distributed-knapsack/135

https://community.spacemesh.io/t/transaction-selection-a-new-idea/185/5

spacemeshos/SMIPS#37

We have a working implementation that’s “good enough for now”. The need for good transaction selection only arises when the mempool is not flushed in every layer.

For now, this work is low priority and will only arise when have a working VM