C++ Client: avoid deprecated Arrow methods, plus conform to coding style

[kosak]

The changes in this PR are:

1. Change a bunch of parameter names to conform to the style guide
2. Change our GetFlightStreamReader method to return a unique_ptr rather than shared_ptr (we were inadvertantly "upgrading" Arrow's returned unique_ptr to a shared_ptr)
3. Reorder the two OkOrThrow methods in arrow_util.h so one can actually successfully call the other (they were in the wrong declaration order, so one couldn't see the other).
4. Change a couple of Arrow calls (DoGet and Next) to use the value-returning versions rather than the deprecated versions with out parameters)
5. Remove a needless using namespace std;