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ARROW-5747: [C++] Improve CSV header and column names options

The `header_rows` option is removed (it seemed untested and probably didn't work well)
and replaced with a `skip_rows` option that dictates how many rows are skipped at
the start of the CSV file (default 0).

Furthermore, a `column_names` option is added to hardcode the column names.
If empty (default), they are read from the first non-skipped row in the CSV file.

Closes #4898 from pitrou/ARROW-5747-csv-column-names and squashes the following commits:

f42840f00 <Antoine Pitrou> Fix downcast
d1381b3bd <Antoine Pitrou> Implement skip_rows + column_names in R
36f2743c5 <Antoine Pitrou> Fix expected semantics for skip_rows
410805254 <Antoine Pitrou> ARROW-5747:  Improve header and column names options

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>