Part of cluster mempool: #30289

[Pitan1993]

Part of cluster mempool: #30289

This builds on #31363, adding more functionality to the txgraph module, specifically:

• TxGraph::GetMainStagingDiagrams(), a function to obtain feerate diagrams for both the main graph and the staged changes to it, including only the clusters that differ between the two.
• TxGraph::GetBlockBuilder(), a function to obtain an object which can efficiently iterate the chunks of the (main) graph from high to low chunk feerate, allowing each to be skipped or included.
• TxGraph::GetWorstMainChunk(), a function to obtain the last chunk that would be returned by GetBlockBuilder()'s returned object, intended for eviction.

Originally posted by @sipa in bitcoin/bitcoin#31444

[loth14]

https://discord.gg/CfWTvZkt