Improve RetryableMultiRegion

[pingyu]

In #332, we add a parameter preserve_region_results to indicate that RetryableMultiRegion should return response from all regions, or raise the first Error if there is any.

As @andylokandy suggests, for lower module coupling, other than using a parameter to control behavior of RetryableMultiRegion, it would be better that RetryableMultiRegion returns a generic response, and allows downstream plans to extract what they need from the response.