Make nChainTx private, ass facility to update it

Summary:
As per title. This ensures we do not depend on implementation specifics.

Depends on D6303

Test Plan:

ninja all check-all

Started a node and stopped it after letting it run for a while a few times to ensure indexing still works as expected.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6308