[C++][Plasma] Fix memory size when using huge pages in plasma; other code cleanups

[asfimport]

In the original code, 'PlasmaAllocator::SetFootprintLimit' happens before dealing with huge pages, which could change the memory limit. Also I found other dirty code needed to be written in a better way.

Reporter: Siyuan Zhuang / @suquark

PRs and other links:

• GitHub Pull Request #6587

_{Note: This issue was originally created as ARROW-8081. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[jorisvandenbossche]

Plasma has been deprecated (#33077) in Arrow 10.0 and subsequently removed in Arrow 12.0 (#33243). Given Plasma is no longer an existing component, closing this issue.

For reference, see this mailing list discussion around possible alternatives: https://lists.apache.org/thread/lk277x3b9gjol42sjg27bst2ggm5s0j2