Add new post-fork ancestor and descendants limit.

Summary:
Adds a getter that returns the correct default limit based on phonon activation status.

Depends on D5242

Test Plan:
Added unit test.
make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Fabien, jasonbcox

Subscribers: Fabien, jasonbcox, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5243