coinbaseTxns => m_coinbase_txns

Summary:
As per title.

This is done to match Bitcoin Core's naming convention.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3314