speed up png resizer

The PNG resizer was only handling 6 requests per second to the healthcheck URL on our big AWS boxes, before maxing their CPUs.

I have done a bit of investigation and it seems it was spending time putting the PNG on the disk for imagemagic to read back in, plus it was doing some conversion to BufferedImages which was also slow (50ms out of a total of 300ms on my laptop).  So I have taken these out and I will see how this helps the performance.  However even running the tools in a pipeline in the shell is 250ms so I think we've hit a limit given the current settings of the tool.

The healthcheck is quicker partly because it scales the images to a tiny size, so pngquant doesn't have much data to deal with.  When using a more realistic size, PngQuant uses the lion's share of the CPU time.

Avenues to explore might be:
use some other png resizing library or tool - is png resizing fundamentally CPU heavy?
don't resize them ourselves after all
precompute the resized images and store in s3
if the requests are mainly duplicates rather than new images, cache them in S3 rather than recomputing them
reduce the load that the heath check makes on the boxes