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

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

```cpp
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.

Author: Matthias Vallentin <vallentin@icir.org>

Closes #1352 from mavam/topic/plasma-manager-fd and squashes the following commits:

f4d4ad37 [Matthias Vallentin] Make retrieval of manager fd a const operation