set TemporarilyUnavailableException::kRetryBackoff higher

Currently, TemporarilyUnavailableException::kRetryBackoff is set to 100 milliseconds, which doesn't seem like enough delay to give the retry of the operation a good chance to succeed. I recommend we change this value to at least 2 seconds, and keep the number of retries hardcoded at "3". Thus, with linear increasing of the delay for each retry, prior to the last retry the delay will be 6 seconds, which doesn't seem unreasonable for an overloaded system. Control of this delay server-side is important since returning a retriable error immediately with no retrying will typically result in immediate retry by the drivers with no delay.