fetchCarbonsearchResponse should have retry or fallback logic

[azhiltsov]

It either need to retry the query to carbonsearch or fail the parent query if carbonsearch fail to respond in time or carbonsearch dies in the middle of a query.
One of other options could be fallback to other carbonsearch instance (config change needed)

[blysik]

I have carbonzipper setup with essentially two sets of backends that are mirrors. It would be awesome if it would retry if one of the systems was down, and hit the mirror.