Add additional comments to select_statement_utils, fix formatting, add
missing #pragma once and introduce set_internal_paging_size_guard to
set internal_paging in RAII fashion.
Closes#7507
Before this change, internal page_size when doing aggregate, GROUP BY
or nonpaged filtering queries was hard-coded to DEFAULT_COUNT_PAGE_SIZE.
This made testing hard (timeouts in debug build), because the tests had
to be large to test cases when there are multiple internal pages.
This change adds new internal_paging_size variable, which is
configurable by set_internal_paging_size and reset_internal_paging_size
free functions. This functionality is only meant for testing purposes.
