[FR] Send `If-Modified-Since` HTTP header to feed servers

[mtlynch]

What

Since fusion knows the last time it refreshed a server's contents, it can send the If-Modified-Since header. The server can respond with HTTP 304 Not Modified to say that there are no updates since our last query.

Why

• Increases performance for fusion
• Reduces bandwidth consumption
• Is kinder to feed servers

(Optional) How

I'd like to keep refactoring the pull module to the point where we split out the HTTP fetch logic into something that's easy for us to unit test.