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ARROW-6558: [C++] Refactor Iterator to type erased handle

To resolve linkage issues with MSVC and the current abstract base class Iterator

@pitrou

Closes #5391 from bkietz/6558-Refactor-Iterator-to-a-ty and squashes the following commits:

903ce2947 <Benjamin Kietzman> revert change to WriteRecordBatchReader, refactor IteratorEnd to a trait class
b57168782 <Benjamin Kietzman> re-add remove_pointer
01445d8a9 <Benjamin Kietzman> remove <iostream>
a0129c972 <Benjamin Kietzman> add comment to constructor
f73f4d519 <Benjamin Kietzman> address review comments
ca0bdf4a7 <Benjamin Kietzman> ARROW-6558:  Refactor Iterator to type erased handle

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