private Path getRetryStateFile(final Path dir) {
        return Objects.requireNonNull(dir).resolve(RETRY_STATE_FILENAME);
    }
@return The {@link RetryState} after initialising/updating it for the attempt that has just
happened.
stroom-proxy/stroom-proxy-app/src/main/java/stroom/proxy/app/handler/RetryingForwardDestination.java