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ARROW-8072 [Plasma] Add const for plasma protocol

See https://issues.apache.org/jira/browse/ARROW-8072. This could also be a blocker for later refactoring like windows integration, since the new interface will just use const instead, and this confused the reviewers in the past (the git diff failed to present the difference correctly because some changes mixed with the 'const' prefix changes).

I also convert "@<!-- -->note" to "\note". I missed it in a previous PR.

Closes #6579 from suquark/add_const

Authored-by: Siyuan <suquark@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>