ARROW-6046: [C++] Do not write excess varbinary offsets in IPC messages from sliced BinaryArray

[wesm]

Even when creating very small slices of a large array starting from offset 0, we were writing the entire offsets buffer unnecessarily.

[codecov-io]

Codecov Report

Merging #5126 into master will decrease coverage by 22.06%.
The diff coverage is 50%.

@@             Coverage Diff             @@
##           master    #5126       +/-   ##
===========================================
- Coverage   87.62%   65.56%   -22.07%     
===========================================
  Files        1014      552      -462     
  Lines      145892    70712    -75180     
  Branches     1437        0     -1437     
===========================================
- Hits       127843    46359    -81484     
- Misses      17687    24353     +6666     
+ Partials      362        0      -362

Impacted Files	Coverage Δ
cpp/src/arrow/ipc/writer.cc	75.66% <50%> (-17.47%)	⬇️
cpp/src/arrow/util/memory.h	0% <0%> (-100%)	⬇️
cpp/src/gandiva/date_utils.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/memory.cc	0% <0%> (-100%)	⬇️
cpp/src/gandiva/decimal_type_util.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/filesystem/util_internal.cc	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/logical_type.h	0% <0%> (-100%)	⬇️
cpp/src/parquet/hasher.h	0% <0%> (-100%)	⬇️
cpp/src/gandiva/basic_decimal_scalar.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/kernels/boolean.cc	0% <0%> (-100%)	⬇️
... and 704 more

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