main: Implement exponential backoff in the feed fetcher loop

Backoff with an exponentially increasing delay when feeds can't be
fetched or parsed, with a maximum of 6h (after which retry every 6h)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "matrix-feedbot"
-version = "0.1.3"
+version = "0.1.4"
 edition = "2021"
 license = "GPL-3.0-or-later"
 authors = ["mirsal <mirsal@mirsal.fr>"]