rpc: Avoid useless mempool query in gettxoutproof

Summary:

GetTransaction implicitly and unconditionally asks the mempool global for a transaction. This is problematic for several reasons:

• gettxoutproof is for on-chain txs only and asking the mempool for on-chain txs is confusing and minimally wasteful
• Globals are confusing and make code harder to test with unit tests

Fix both issues by passing in an optional mempool. This also helps with core#19556

This is a backport of core#19589

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9783