Fix uninitialized atomic variables

Summary:
Backport of PR10819
https://github.com/bitcoin/bitcoin/pull/10819/files

Note that only the changes to src/wallet/db.h are applicable, the
others are already part of our codebase.

Completes T658

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3096