This commit adds a cache for 'getContents' to '/cache/server'. All
contents are cached by default (even in debug mode). If debug mode
is enabled, the cached data is overwritten on each request.
In normal mode RSS-Bridge adds the 'If-Modified-Since' header with
the timestamp from the previously cached data (if available) to the
request.
Find more information on 'If-Modified-Since' here:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since
If the server responds with "304 Not Modified", the cached data is
returned.
If the server responds with "200 OK", the received data is written
to the cache (creates a new cache file if it doesn't exist yet).
No changes were made for all other response codes.
Servers that don't support the 'If-Modified-Since' header, will
respond with "200 OK".
For servers that respond with "304 Not Modified", the required band-
width will decrease and RSS-Bridge will responding faster.
Files in the cache are forcefully removed after 24 hours.
Notice: Only few servers actually do support 'If-Modified-Since'.
Thus, most bridges won't be affected by this change.
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