Make m_coinbase_txns a vector of CTransactionRef

Summary:
This is extracted from Core PR12949, but the PR doesn't apply cleanly, so we'll have to redo it manually.

Depends on D3314

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3315