Feature request: Add config option for matrix resend backoff

[a-0-dev]

I kind of rely on the RSS bot to send messages of a feed into a room, however some messages are dropped if there are errors (such as the homeserver not responding or similar). It would be great if there was a (configurable) resend backoff which tries to send the matrix messages again if some error occured.

Suggestions:

• Exponential backoff with maximum of 4 weeks or so
• Hard-coded resend interval of e.g. 4 hours
• Configurable by one integer (try again after n hours), maximum 4 weeks or so

This would also cover potential issues if there is any other problem with the bot which is fixed few weeks after, because feed items which weren't processed properly (e.g. ending in a matrix message) would be processed again until it worked or the absolute limit of suggested 4 weeks is reached.