Use range based for loop

Summary:
Instead of iterating over 0 .. 1 and then deciding on an actual desired
value, use a range based for loop for the desired value.

Backport of core PR10844

Test Plan:

./src/test/test_bitcoin --run_test=dbwrapper_tests

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2200