Exponential reconnect backoff

When fetch-event-source loses it's connection to the server, it tries to reconnect rather aggressively. Given enough clients, this continious repeating traffic pattern may pose an unneccesary risk of overloading a router/ load balancer that might still be receiving the traffic.


This issue proposes to implement an exponential backoff behavior that increases the reconnect time with each unsuccessful reconnect iteration (see https://www.npmjs.com/package/exponential-backoff for an example implementation)