Activate CSV based on height rather than BIP9

Summary: It's not liek we expect to have a reorg deep enough to change anythign about its activation by now.

Test Plan:

make check

Run bip68-112-113-p2p.py

Run full IBD on mainet and testnet with flags -assumevalid=0 -nocheckpoints

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1531