ARROW-1852: [C++] Make retrieval of Plasma manager fd a const operation

[mavam]

This small patch makes it possible to retrieve the manager fd in a const context. For example:

class handle {
public:
  bool connected() const {
    return client_.get_manager_fd() != -1;
  }
private:
  plasma::PlasmaClient client_;
};

Without this patch, it's impossible to implement the above function.

An alternative would be to simply provide such a utility function as part of PlasmaClient, but this changes the API. Let me know what you prefer.

[pcmoritz]

+1 LGTM! I guess we can merge this without a JIRA via the github ui given how small the patch is?

[mavam]

I saw a bunch of open PRs at github, so I just opened one here. Is JIRA a place to do this?

[wesm]

@mavam see https://github.com/apache/arrow#how-to-contribute

[wesm]

It'd be fine to merge a small patch like this with no JIRA for the changelog, but I created one anyway (@mavam can you let me know your JIRA id so I can assign this to you?) https://issues.apache.org/jira/browse/ARROW-1852

[mavam]

(@wesm My JIRA username is the same as my github handle: mavam.)

[wesm]

Hm, I'm not finding you on the Apache JIRA

[pcmoritz]

Great, thanks for the contribution and for merging :)