New Schnorr multisig activation
Summary:
Based on D2483, but the functional test uses the new features recently
introduced (see current abc-schnorr.py).
Depends on D3474
Test Plan:
make check
test_runner.py
test_runner.py --with-gravitonactivation
Reviewers: deadalnix, Mengerian, jasonbcox, Fabien, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D3736