wallet: Avoid leaking nLockTime fingerprint when anti-fee-sniping
Summary:
The wallet sets the locktime to the current height of our active chain. This is fine, as long as our node is connected to other nodes. However, when we fall back and get stuck at a particular height (e.g. taking the wallet offline), the same (potentially unique) locktime is used for all transactions. This makes it easier for passive observers to cluster transactions by wallet.
Backport of core PR15039.
Test Plan:
ninja check check-functional-extended
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D5560