Only cache Streamers with basic configuration

[minborg]

There is an infinite number of potentiall StreamConfiguration so we should limit the cache size.

[minborg]

This is a potential fix:

    @Override
    @SuppressWarnings("unchecked")
    public <T> Stream<T> stream(final StreamConfiguration<T> streamConfiguration) {
        requireNonNull(streamConfiguration);
        if (streamConfiguration.joins().isEmpty()) {
            // Only cache simple configuration to limit the number of objects held
            // See https://github.com/speedment/jpa-streamer/issues/56
            return (Stream<T>) streamerCache
                    .computeIfAbsent(streamConfiguration, ec -> new StandardStreamer<>(streamConfiguration, entityManagerFactory))
                    .stream();
        } else {
            // The StandardStreamer::close method never gets called.
            return new StandardStreamer<>(streamConfiguration, entityManagerFactory).stream();
        }
    }

However, the The StandardStreamer::close method never gets called so the renderer remains open.

[minborg]

We could add a close handler to the stream itself.