Add activation code for SEGWIT_RECOVERY
Summary:
As per title.
Depends on D2468 and D2527.
Test Plan: test_runner.py abc-segwit-recovery-activation
Reviewers: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, florian
Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc, jasonbcox
Subscribers: Fabien, Mengerian, markblundeberg, jasonbcox, teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2479