PARQUET-1160: [C++] Implement BYTE_ARRAY-backed Decimal reads

[thaining]

This change enables support for DECIMALs backed by BYTE_ARRAYs on disk. It does this by creating a TransferFunctor routine that transforms a ByteArrayType to an ::arrow::Decimal128Type.

The routine does this by:

1. Creating an arrow::BinaryArray from the RecordReader's builder
2. Allocating a buffer for the arrow::Decimal128Array
3. Converting the big-endian bytes in each BinaryArray entry to two integers
   representing the high and low bits of each decimal value.

[wesm]

rebased

[xhochy]

@thaining ReadDecimals/TestArrowReaderAdHocSparkAndHvr.ReadDecimals/4 is failing in CI, can you have a look what's going wrong?

[thaining]

@xhochy, I am able to reproduce the problem locally by running 'ctest -L parquet'. The problem is that the SHA for cpp/submodules/parquet-testing does not have the file byte_array_decimal.parquet needed for the test. If I manually add that parquet file to $ARROW_ROOT/cpp/submodules/parquet-testing/data, the test passes.

When I do "git submodule update --init" on the arrow tree, it leaves the submodule at "HEAD detached at 48a657c". That's the wrong SHA. If I switch the submodule to the latest trunk, I get SHA 46ae260, which has the file. The test also passes.

[xhochy]

+1, LGTM

Build failure is unrelated (wheel/multibuild incompability)