This refactor ticket aims to split KeysCollectionCacheReaderAndUpdater into KeyGenerator and KeysCollectionCacheReader. To achieve this, we need to:

• Remove existing KeysCollectionCache.
• rename KeysCollectionCacheReader to KeysCollectionCache
• rename KeysCollectionCacheReaderAndUpdater to KeyGenerator
• change existing code in KeysCollectionManager to always have a KeysCollectionCache and create a KeyGenerator in here and destroy in here

change KeysCollectionCacheReaderAndUpdater to the following api:

class KeyGenerator {
    Status generateNewKeysIfNeeded(OperationContext* opCtx);
    // removed the getKey methods, existing callers should call the KeysCollectionCache instance instead.
};