`indexed_headers` should be locked during the whole process of `stats` function

In the function `stats` in `schema.rs`, `indexed_headers` is used both in checking the validity of the cache (in `height_by_hash`) and updating the stats (in `stats_delta->tx_confirming_block`).

We should keep `indexed_headers` locked during the whole process of `stats`. Otherwise, `indexed_headers` can be changed inside the `stats` function, which will lead to the cache and updated stats being build based on different blockchains.

Although the probability is really low for `indexed_headers` to be changed during the running of the `stats` function, it will also happen, especially for address with a lot of transactions whose `stats_delta` will last a long time.