Introduce `CoinsViewOverlay`, a `CCoinsViewCache` subclass that reads
coins without mutating the underlying cache via `FetchCoin()`.
Use `PeekCoin()` to look up a Coin through a stack of `CCoinsViewCache` layers without populating parent caches. This prevents the main cache from caching inputs pulled from disk for a block that has not yet been fully validated. Once `Flush()` is called on the view, these inputs will be added as spent to `coinsCache` in the main cache via `BatchWrite()`.
This is the foundation for async input fetching, where worker threads must not
mutate shared state.
Co-authored-by: l0rinc <pap.lorinc@gmail.com>
