Allows making vExtraTxnForCompact and vExtraTxnForCompactIt member vars
instead of globals.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/21148 | core#21148]] [13/14]
https://github.com/bitcoin/bitcoin/pull/21148/commits/eeeafb324ef6057f40b5c5fdd8464110e809b0f7
Depends on D11491