block index: split ResetChainStats from UpdateChainStats

Summary:
In D6308 the tasks of resetting nChainTx to 0 (for new blocks that don't yet have transactions downloaded) or to it's correct value (for connected blocks) was factored into a single UpdateChainStats function.

In a following backport of https://github.com/bitcoin/bitcoin/pull/29370/commits/ef29c8b662309a438121a83f27fd7bdd1779700c, I will have to make the resetting in ChainstateManager::ReceivedBlockTransactions conditional on the block not being a pruned block which is being downloaded again and not being the assumeutxo snapshot's base block. In BlockManager::LoadBlockIndex the resetting will be done under slighty different conditions (having parent blocks with missing transactions and not being the snapshot's base block)

Prepare for this change of behavior by splitting the resetting code from the updating code, and introducing a MaybeResetChainStats wrapper that will check for the conditions before maybe calling ResetChainStats

Depends on D17878

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17872