FindKey finds key that is greater than or equal to the given key. It returns ErrNotFound if the table doesn't contain such key. If filtered is true then the nearest 'block' will be checked against 'filter data' (if present) and will immediately return ErrNotFound if 'filter data' indicates that such key doesn't exist.
The caller may modify the contents of the returned slice as it is its own copy. It is safe to modify the contents of the argument after Find returns.