[C++] Refactor scan_node to introduce support for scan tasks

[westonpace]

Describe the enhancement requested

This is a prerequisite task for adding support for parquet to the new scan node. The current implementation has two problems that I am seeking to improve. It is too focused on CSV and did not account for a row-group-like concept. The second is that the sequencing logic is scattered throughout the scan node implementation which makes it difficult to follow.

Component(s)

C++