Enforce LOW_S after Nov, 13

Summary: This is something that was in the plans, so let's just do it.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

NB: prioritise_transaction.py fails but it doesn't looks related to this diff.

Reviewers: schancel, freetrader, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D616