refactor: Add and use HaveTxsDownloaded() where appropriate

Summary:
nChainTx is an implementation detail that shouldn't be exposed without a wrapper that comes with appropriate documentation.

Backport of Bitcoin Core PR14863
https://github.com/bitcoin/bitcoin/pull/14863

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4552