Fix initialization of setBlockIndexCandidates when working with multiple chainstates

Summary:
When using assumeutxo and multiple chainstates are active, the background
chainstate should consider all HAVE_DATA blocks that are ancestors of the
snapshotted block and that have more work than the tip as potential candidates.

This is a partial backport of core27746
https://github.com/bitcoin/bitcoin/pull/27746/commits/768690b7ce551cd403f8e2a099372915f6022ad4

Depends on D17534

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17535