querier: improve chunkseries iterator

[MichaHoffmann]

Accessing the current chunk via the slice is expensive and we should generally prefer to call methods on the memoized "cur" here. Moved the AtT at the bottom - an exhausted iterator should generally still be at its last value there so this should be sound - this removes a lot of AtT function call overhead. Did same with the Err() call as thats generally only interesting if we are not the last chunk and was surprisingly hot in profiles.

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