Schnorr signature activation

Summary:
Includes python activation tests based on replay protection activation.

Features:

• Tests that when upgrade / downgrade happens, newly-invalid transactions are dumped from mempool (but not valid ones).
• Don't ban peers who send a tx that would be valid under the mandatory flags on other side of upgrade/downgrade.
• Tests of 64-byte DER signatures

Test Plan:
run functional tests
uncomment fakeDER64 and patch VerifySignature (see comment in file about fakeDER64)
run abc-schnorr-activation.py again

(I have also run these tests with the various critical bits of mainline code cancelled out, and verified the tests fail as expected.)

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox, Fabien

Subscribers: Fabien, jasonbcox, florian, Mengerian, teamcity, schancel

Maniphest Tasks: T527

Differential Revision: https://reviews.bitcoinabc.org/D2483