[C++][Parquet] Use arrow::Buffer for buffered dictionary indices in DictEncoder instead of std::vector

[asfimport]

Follow up to ARROW-6411

Reporter: Wes McKinney / @wesm

Related issues:

• [C++][Parquet] DictEncoderImpl::PutIndicesTyped has bad performance on some systems (relates to)

_{Note: This issue was originally created as PARQUET-1646. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
While on this task, it might be good to avoid deallocating memory in between dictionary page writes (currently the vector is being cleared). 

[github-actions]

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