Implement 304 response to If-Modified-Since header

[flipmcf]

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since

When a requests for a playlist or segment includes the "If-Modified-Since" header, and that video is still cached in casterpak, reply with a 304 and update CaterPak's TTL int the cache database.

This is basically a signal that the video is still being accessed by viewers and should remain in the cache.

If the file is not cached, reply with a 200 and the payload (playlist or segment) as normal.

Upstream caches, such as a CDN, should use this If-Modified-Since request header to signal to CasterPak that a video is still being played and should remain in CasterPak's cache, but the requests are being handled by upstream caches.