Request headers to bypass cache

[Deleplace]

Current behavior:

• pages are cached server-side (Memcache)
• cached versions are served until they expire (or are evicted, full cache is flushed)

Desired behavior:

• pages are cached server-side (Memcache)
• if request header cache-control is no-cache or max-age:0, then don't use the Memcached version.