Implenting very rudimentary http caching

POC

adds ETAG and Last-Modified headers
rudimentary handling of incoming if-none-match and if-modified-since

Currently only enabled if memcach cache is enabled
If I understand this correctly, we should be able to check if the client sent over an etag here, and if so, return a 304 response as well.

I don't think that the dates will help us though, because we don't have insight into when the content for a tile changed at the moment. If it's not in memcache, then we'll just report back a new date. Maybe we should just support etags for now?