[Backport] Fix four bugs with numeric dimension output types.

[gianm]

This patch includes the following bug fixes:

• TopNColumnSelectorStrategyFactory: Cast dimension values to the output type
  during dimExtractionScanAndAggregate instead of updateDimExtractionResults.
  This fixes a bug where, for example, grouping on doubles-cast-to-longs would
  fail to merge two doubles that should have been combined into the same long value.
• TopNQueryEngine: Use DimExtractionTopNAlgorithm when treating string columns
  as numeric dimensions. This fixes a similar bug: grouping on string-cast-to-long
  would fail to merge two strings that should have been combined.
• GroupByQuery: Cast numeric types to the expected output type before comparing them
  in compareDimsForLimitPushDown. This fixes ClassCastException in groupBy when sorting on numeric columns containing nulls #6123.
• GroupByQueryQueryToolChest: Convert Jackson-deserialized dimension values into
  the proper output type. This fixes an inconsistency between results that came
  from cache vs. not-cache: for example, Jackson sometimes deserializes integers
  as Integers and sometimes as Longs.

And the following code-cleanup changes, related to the fixes above:

• DimensionHandlerUtils: Introduce convertObjectToType, compareObjectsAsType,
  and converterFromTypeToType to make it easier to handle casting operations.
• TopN in general: Rename various "dimName" variables to "dimValue" where they
  actually represent dimension values. The old names were confusing.

• Remove unused imports.