No behavior change.

The goal here is to group indexes, so we can perform the same
initialization and verification process equally for all of them.

The checks performed inside StartIndexes will be expanded
in the subsequent commits.

This is a partial backport of core#27607
https://github.com/bitcoin/bitcoin/pull/27607/commits/225e213110602b4fd1d345167f5f92d26557f6c1
Depends on D17745

Note that we need to return false if StartIndexes fails or else tests will fail because the node will not abort on failure to start an index. This code divergence from Core goes away in the next commit.

ninja all check check-functional-extended