Give a proper value for the anti replay comitment

Summary:
Set the actual value of the anti replay commitment.

I also had to change the value of the padding, or we generate an invalid coinbase.

Test Plan:

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

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

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D281