Do some local disk caching for site fetches (as well as memcached)

[simonbaird]

As well as memcached I think a file system cache would also be useful to cache S3 fetches.

Reasoning:

• Sites are large so in memory cache needs a lot of memory and we don't have a lot. But we could easily add a few G of disk cache.
• The benefit of caching to local disk (vs read from S3 everytime) would be significant, especially for popular sites that don't change much.

[simonbaird]

I have a better idea of what's going on with memcached now, so I'm less worried about it. But I think the local disk cache is still a good idea, so I'll rename and keep it open.