Wrap nChainTx into GetChainTxCount

Summary: This ensure that we can change the layout underneath while keeping the same API.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6303