Like monolith upgrade, this one was possible to make retroactively
always active. After this change, "great wall" is only mentioned in
the chainparams.cpp checkpoint comment.
Depends on D3253 and D3194
test_runner.py (to show nothing was using -greatwallactivationtime)
Build Bitcoin-ABC / Diffs / Bitcoin-ABC Diff Testing started.
Build Bitcoin-ABC / Diffs / Bitcoin-ABC Diff Testing passed.
Looks good to me.
rebase onto split parent