ARROW-5626: [C++] Fix caching of expressions with decimals

[pravindra]

• use full DataType instead of just name
• added test for caching decimal expr

[praveenbingo]

+1 LGMT.

[fsaintjacques]

The fix/implementation depends on the Type::ToString method to return a unique identifier based on the (runtime) parameters. This is an implicit contract. I think it's worth adding a class that is simply a TypeVisitor that returns a key and properly warn when a type is unknown (and not cachable).

[codecov-io]

Codecov Report

Merging #4592 into master will decrease coverage by 13.25%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #4592       +/-   ##
===========================================
- Coverage   88.57%   75.31%   -13.26%     
===========================================
  Files         860       56      -804     
  Lines      108022     3192   -104830     
  Branches     1253        0     -1253     
===========================================
- Hits        95678     2404    -93274     
+ Misses      12065      788    -11277     
+ Partials      279        0      -279

Impacted Files	Coverage Δ
python/pyarrow/ipc.pxi
cpp/src/arrow/csv/chunker-test.cc
cpp/src/parquet/column_page.h
cpp/src/parquet/bloom_filter-test.cc
cpp/src/arrow/array/builder_decimal.cc
cpp/src/plasma/client.cc
cpp/src/arrow/io/test-common.h
cpp/src/arrow/util/int-util-test.cc
cpp/src/arrow/python/io.cc
python/pyarrow/hdfs.py
... and 790 more

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[pravindra]

The fix/implementation depends on the Type::ToString method to return a unique identifier based on the (runtime) parameters. This is an implicit contract. I think it's worth adding a class that is simply a TypeVisitor that returns a key and properly warn when a type is unknown (and not cachable).

agree - we've already hit a couple of bugs related to this. I'll open a jira for this.