Redundant volatiles in BaseTopNAlgorithm.BaseArrayProvider

[leventov]

https://github.com/druid-io/druid/blob/master/processing/src/main/java/io/druid/query/topn/BaseTopNAlgorithm.java#L176-L179

Similar to situation here: #3797 (comment)

I expect that at most one field should remain volatile. Or none (probably a little refactoring needed).

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