Exponential backoffs for all connection attempts

[realcr]

There are some parts in offst that attempt connections:

• Relay client
• Channeler
• Index client

Currently each of those have a very basic backoff mechanism for reconnection after a failed connection attempt. This might cause DoS on a server.

We should change the backoff mechanism to exponential backoff, and find reasonable ways to test the new exponential backoff logic. Maybe there is a way to isolate this logic and reuse it in all clients that connect to servers in offst.