PTFM-13951 Retry upon connection error even for non-idempotent requests

@kislyuk, care to review?
Something to consider, but not necessarily do anything about, is that the new behavior of retrying on name resolution or connection refused errors may be annoying in interactive use when dx is misconfigured. Perhaps it's worth it to make it configurable or even auto-configure it not to retry if sys.stdin.isatty() (but I'm not convinced of that need yet).