Backport of #4160 to release-v2.0.x.
- adds command.WasLeaderless to classify cluster-has-no-leader dispatch errors as retriable
- replies busy instead of dropping or hard-failing terminator creates when the cluster is briefly leaderless, so the router backs off and requeues promptly rather than waiting for its multi-minute recovery scan
- removes the racy up-front leaderless pre-check in the sdk create handler in favor of classifying the actual dispatch result
- applies the same retriable classification to the ert tunnel create and batch remove terminator handlers
Paul Lorenz
c0bf5b7126