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ARROW-3666: [C++] Improve C++ parser performance

Make CSV parsing around 30% faster.

Benchmark of reading a CSV file with integer columns. Before:
* multi-threaded: 1.2 GB/s
* single-threaded: 190 MB/s

After:
* multi-threaded: 1.6 GB/s
* single-threaded: 210 MB/s

(on a 8-core 16-thread AMD Ryzen CPU)

Author: Antoine Pitrou <antoine@python.org>

Closes #2886 from pitrou/ARROW-3666-faster-csv-parser and squashes the following commits:

1c60c54b <Antoine Pitrou> Address review comments
91216a35 <Antoine Pitrou> ARROW-3666:  Improve C++ parser performance