Add Exponential Backoff mechanism

[gkorland]

The current retry mechanism is too aggressive for SQL Azure.
Too many consecutive retries get "punished" by SQL Azure.

See:

1. Retry service specific guidance
   
2. Transient Fault Handling (Building Real-World Cloud Apps with Azure)

[seanlindo]

@gkorland I want to make sure I've understood the issue correctly-- is it possible that some users would see wait times for whatever action they're taking in the UI reach minutes?

[gkorland]

@seanlindo no of course not but when a pool is retrying to fill the minimum connections it should use this retry gracefully.

[ben-page]

I'll try to find some time to look over this. But, I'd definitely accept a PR for this.