[Plasma] Use Scope enum in Plasma

[asfimport]

Modernize our usage of enums in plasma:

1. add option "--scoped-enum" to Flat Buffer Compiler.
2. change the old-styled c++ enum to c++11 style.

Reporter: Philipp Moritz / @pcmoritz
Assignee: Yuhong Guo / @guoyuhong

PRs and other links:

• GitHub Pull Request #2179

_{Note: This issue was originally created as ARROW-2758. Please see the migration documentation for further details.}

[asfimport]

Philipp Moritz / @pcmoritz:
Issue resolved by pull request 2179
#2179

[asfimport]

Yuhong Guo / @guoyuhong:
Hi Philipp, this is Yuhong. Please assign this task to me. Thanks!

[asfimport]

Uwe Korn / @xhochy:
@guoyuhong Assigned you and gave you the necessary karma to self-assign tickets in future. Thanks for contributing.