[Chronik] Add GroupHistoryWriter::member_num_pages_and_txs

Summary:
Replace member_num_txs in GroupHistoryWriter with member_num_pages_and_txs, as we often need both the number of pages and txs of a member at the same time, and we can read them in one go without going back to the DB.

We also add some tests (previously member_num_txs was untested).

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13561