HTTPSubscriber: check with If-None-Match or If-Modified-Since

[lidel]

There are two ways HTTP servers (incl. HTTP gateway responses for /ipns paths) may support update checks: via Etag or Last-Modified headers. We could improve HTTPSubscriber to maximize cache hits, and if Etag or Last-Modified are present in the response, store them, and use in follow-up update checks:

• if Etag was present, check for update with If-None-Match
• if Etag was not present, but Last-Modified was present, check for update with If-Modified-Since
  • (we prefer If-None-Match, because Etags are more deterministic than time-based check)

This is a bit nicer because if the content did not change since the last check, the response will be HTTP 304 Not Modified without any payload, rather than HTTP 5XX error, and will be produced by edge cache/cdn, rather than hitting the backend every time.