This refactor unlocks some nice potential features, such as, but not limited to:
- Removing the fee estimates global (would avoid slightly fragile workarounds such as Disable fee estimation in blocksonly mode (by removing the fee estimates global) #18766)
- Making the mempool optional for a "blocksonly" operation mode
Even absent those features, the new code without the global should be easier to maintain, read and write tests for.
This is a backport of core#19556 [1/3]