Resize images asynchronously

[gerdint]

Yields a 2x speed-up on my dual-core i5 machine.

However, as it stands each size of an image to be resized yields a new subprocess potentially spawning many subprocesses concurrently if the site contains a lot of images (such as my own). This increases memory usage and may therefore cause disk swapping if memory is limited. Does not seem to be a big issue on my 8GB SSD-equipped machine but I understand performance characteristics may be different on other machines.

It may perhaps be preferable to a have a job server making sure that no more than N processes are spawned concurrently. A useful but potentially conservative value for N may be the number of processor cores.

OR perhaps I'm over-engineering here. WDYT?