Merge #11422: qa: Verify DBWrapper iterators are taking snapshots

Summary:
bb8376b Verify DBWrapper iterators are taking snapshots (Matt Corallo)

Pull request description:

The LevelDB docs seem to indicate that an iterator will not take
snapshots (even providing instructions on how to do so yourself).
In several of the places we use them, we assume snapshots to have
been taken.

In order to make sure LevelDB doesn't change out from under us
(and to prevent the next person who reads the docs from having the
same fright I did), verify that snapshots are taken in our tests.

Tree-SHA512: 54f24dabc294962e9c20882f61809604421a661208d1568bb107102248603e8e7c12e929ccb0812a73d4e4f23fea61f1b48e7cc24da5a7260f1d14d89ba88cd6

Backport of Core PR11422
https://github.com/bitcoin/bitcoin/pull/11422/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3858