LoadBalancer should not fail wait-for-ready requests when there is an errror

[zhangkun83]

Currently LoadBalancer would just call TransportManager.InterimTransport.closeWithError(), which fails all requests. We should instead add another method like enterTransientFailure(), which is reversible by exitTrasientFailure() and only kills current and further fail-fast requests.

closeWithError() shall be kept for shutdown, or be renamed to closeWithShutdown().

[ejona86]

@zhangkun83, this looks like it was fixed with LBv2 API. Agreed?

[zhangkun83]

Yes, it was.