move-only: Define TransactionNotification before TransactionTablePriv

Summary:
This is needed because next commit moves vQueueNotifications to TransactionTablePriv member.

This is a backport of core-gui#120 [1/2]
https://github.com/bitcoin-core/gui/pull/120/commits/7b3b2303f44031c3545651858f697a495c3ea37a

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10730