* Make SessionTracker heartbeat loop more robust
`(*SessionTracker).UpdateExpirationLoop` terminated the first time
it got an error attempting to persist the updated expiry to Auth.
A momentary network outage could prevent the `types.SessionTracker`
from being updated again causing it to have the wrong TTL in the
backend. This could result in sessions sticking around longer than
needed or being assumed to have been terminated prior to the session
actually ending.
A `retryutils.Retry` has been added to allow for retrying to update
a `types.SessionTracker` with some backoff. The retry loop will continue
until the session is closed, the update is successful, or the retry
has occurred for longer than the `types.SessionTrackers` last known
expiry.
`apidefaults.SessionTrackerTTL` was also reduced from 1h to 30m to
reduce the amount of time a stale tracker may linger in the backend.
The added robustness to the update loop should alleviate the need for
the lengthy TTL.
rosstimothy