[FR]: Support "e-tag" for all network requests

[martinrotter]

Brief description of the feature request

Add some kind of logging of "e-tag" HTTP headers and reuse them for subsequent RSS queries to given URL.

If feed is not updated since last time, then server returns HTTP/304 which should be recognized by RSS Guard as "no new messages" - this is much faster than redownloading articles all the time repeatedly.

This can be likely added to almost all network communication regarding downloading of feeds and even into specialized plugins like gmail - greatly speeding up the process.

https://fishbowl.pastiche.org/2002/10/21/http_conditional_get_for_rss_hackers¨

EDIT: Added so far only for standard feeds.

[martinrotter]

From initial testing, speed up with 100 feeds is about 12-18 %, which is worth it I guess.