validation: run VerifyDB on all chainstates

Summary:

refactor: rename active_chainstate in VerifyDB

To prepare VerifyDB semantics for multiple chainstate use.

validation: prepare VerifyDB for assumeutxo

Removes assumptions of use only on the active chainstate.

doc: IsSnapshotActive

This is a backport of core#21523

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11334