Add function 'IsGreatWallEnabled'

Summary: This checks whether network upgrade code named 'great wall' has activated.

Test Plan: Added test

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: markblundeberg, jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2375