A couple tests require lowering the grace period to keep the test from
taking the full 30s timeout.
The Retry_hang test also needed to be removed from the Parallel group,
becuase it modifies the global refreshGracePeriod variable.
Refresh calls may have side effects that need to be recorded if it
succeeds, especially common when when WaitForState is called from
resource.Retry.
If the WaitForState timeout is reached and there is a Refresh call
in-flight, wait up to refreshGracePeriod (set to 30s) for it to
complete.
Change the `RetryFunc` from a plain `error` return type to a
specialized `RetryError` which must decide whether it is
retryable or not.
Add `RetryableError` / `NonRetryableError` factory functions that
callers are meant to use to build up these errors.
This makes it eminently clear whether or not a given error is
retryable from inside the client code.
Goal here is to _not_ change any behavior, simply reflect the
existing behavior with the new, clearer, API.
James Bardin