timeutils function retry_backoff_delay

Calculates an exponential backoff delay with jitter for retrying
operations which might fail.

The exponential delay on each attempt gives the service a chance
to process its backlog of retries after it starts responding again.

The jitter adds some randomness to the delay to prevent
multiple synchronised backoff retries from hitting the
service at exactly the same time.

The next change in this series makes use of this function.

Change-Id: Ie209ef62c0da68a5b503354764e5add2b6e5569a
Merge "timeutils function retry_backoff_delay"