Incorrect exponential backoff algorithm in RetryConfigFromDeliverySpec()

[travis-minke-sap]

Describe the bug
The default exponential backoff algorithm defined in RetryConfigFromDeliverySpec() is incorrect and does not implement the advertised calculation from DeliverySpec.BackoffDelay. Instead of implementing backoffDelay*2^<numberOfRetries> as advertised, it is coded as (backoffDelay*2)^<numberOfRetries>.

Not only does this not operate as expected, but results in negative Durations due to the fact that time.Duration is stored in nanoseconds, and raising such large numbers to exponential powers quickly exceeds the int64 size of time.Duration, causing a roll-over to negative values.

Expected behavior
The algorithm should be implemented as documented and not roll-over to negative time.Duration values.

To Reproduce
Create a Subscription which specifies the following spec.delivery...

spec:
  delivery:
    backoffDelay: PT2S
    backoffPolicy: exponential
    retry: 5

...and attempt to send events to a channel which utilizes RetryConfigFromDeliverySpec() and DispatchMessageWithRetries().

Knative release version
v0.17.4

[travis-minke-sap]

/assign @travis-minke-sap