Allow setting nMinimumChainWork on command line

Summary:
Backport core's PR10357

Depends on D1129

Test Plan:

make check
./test/functional/test_runner.py minchainwork

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1130