Remove mempool global from init

Summary:
PR description:

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]
https://github.com/bitcoin/bitcoin/pull/19556/commits/eeee1104d78eb59a582ee1709ff4ac2c33ee1190

Depends on D9780, D9783 and D9784

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9788