Add function 'IsGravitonEnabled'

Summary:
This function checks whether the Nov 15, 2019 Upgrade, code named
'Graviton', has activated

Based on D2375

Test Plan:
make check
I also experimented with the unit test by changing various parameters,
and ensuring it fails when it should.

Reviewers: deadalnix, Fabien, jasonbcox, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2878