Replace CommitInternal method with CustomCommit and use interfaces::Chain
instead of CChainState to generate block locator.
This commit does not change behavior in any way, except in the
(m_best_block_index == nullptr) case, which was added recently in
D12622 as part of an ongoing attempt to
prevent index corruption if bitcoind is interrupted during startup. New
behavior in that case should be slightly better than the old behavior (skipping
the entire custom+base commit now vs only skipping the base commit previously)
and this might avoid more cases of corruption.
This is a partial backport of core#25494
https://github.com/bitcoin/bitcoin/pull/25494/commits/7878f97bf15b6e7c9b47d1c0d96419b97e1bdcbd