Merge #12225: Mempool cleanups

Summary:
669c943 Avoid leaking prioritization information when relaying transactions (Suhas Daftuar)
0975406 Correct mempool mapTx comment (Suhas Daftuar)

Pull request description:

Following up on #12127 and #12118, this cleans up a comment that was left incorrect in txmempool.h, and addresses a couple of the observations @TheBlueMatt made about an unnecessary use of `queryHashes()` and a small information leak when prioritizing transactions.

Left undone is nuking queryHashes altogether; that would require changing the behavior of the `getrawmempool` rpc call, which I think I might be in favor of doing, but wanted to save for its own PR.

Tree-SHA512: c97d10b96dcd6520459287a4a2eda92774173757695100fcfe61e526aef86f394507c331d17f9e0c14b496c33ec46198a0f165a847762ca50f7c6780b993f162

Backport of Core PR12225
https://github.com/bitcoin/bitcoin/pull/12225/

Depends on D3711

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3789