Step off retries

[nextmat]

We should improve our current retry logic to delay in a gradual step-off pattern, something along the lines of:

https://github.com/eric/metriksd/blob/master/lib/metriksd/librato_metrics_reporter.rb#L56-L69

[findchris]

👍 Definitely a good idea for resilience.
While we are looking at this bit of code, I'd like to see this persistence logic be able to flush asynchronously, either in a thread or, if using EventMachine, using EM.next_tick.