[backport#15842] refactor: replace isPotentialtip/waitForNotifications by higher method

Summary:
Add GUARDED_BY(cs_wallet) annotation to m_last_block_processed, given
that its now guarded by cs_wallet instead of cs_main
https://github.com/bitcoin/bitcoin/pull/15842/commits/422677963a7b41e340b911b4cd53d29dd8d63f21

Add WITH_LOCK macro: run code while locking a mutex
Results from ryanofksy suggestion on isPotentialTip/
waitForNotifications refactoring
https://github.com/bitcoin/bitcoin/pull/15842/commits/edfe9438ca54274815f076fc21beb790df5aa0a2

Depends on D6258

Backport of Core PR15842

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6260