Use Median Past Time for HF activation

Summary:
For all intent and purpose they are the same as long as CSV is activated, which it has been for ages. For it is different when running integration tests as CSV is not activated there.

Plus the spec specify MPT so we should use that.

Test Plan:

make check

Reviewers: freetrader, awemany, sickpig, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D190