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ARROW-6964: [C++][Dataset] Add multithread support to Scanner::ToTable

The caller may request a parallel construction of the table. Scanner was refactored to own the ScanOptions and ScanContext members.

Closes #5721 from fsaintjacques/ARROW-6964-parallel-scanner-to-table and squashes the following commits:

210f1905f <François Saint-Jacques> ARROW-6964:  Add multithread support to Scanner::ToTable

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>