[fea] Expose the arena mr to the Python interface.

[trivialfis]

Description

Close https://github.com/rapidsai/rmm/issues/830 .

• Add the arena allocator to the public Python interface.
• Small changes to the logger initialization to avoid exposing spdlog in the shared objects.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[wence-]

Thanks!

[vyasr]

/merge