arrow

ARROW-7916: [C++] Project IPC batches to materialized fields only

This is an optimization where if both a filter and a projection are given, we apply the -pre-projection before the filter such that columns not referenced by the final projection are not copied.

Closes #6474 from bkietz/7916-Dataset-Project-IPC-recor and squashes the following commits:

f8f119442 <Benjamin Kietzman> revert to unique-ing materialized fields in IpcScanTask
a1b3362e3 <Benjamin Kietzman> refactor file format tests for clarity
c268e9bc7 <Benjamin Kietzman> remove unique-ing of materialized fields
624036323 <Benjamin Kietzman> ARROW-7916:  Project IPC batches to materialized fields only

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>