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.

make check
test_runner.py