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ARROW-3146: [C++] Prototype Flight RPC client and server implementations

This is a partial C++ implementation of the Flight RPC system initially proposed by Jacques in ARROW-249. As in Java, I had to dig into gRPC and Protocol Buffers internals to ensure that

* On write, memory is only copied once into the outgoing gRPC buffer
* On read, no memory is copied

The way that I tricked gRPC into circumventing the built-in protobuf serde paths might look a bit hacky, but after digging around in the library a bunch I've convinced myself that it's the best and perhaps only way to accomplish this. Luckily, the message that's being serialized/deserialized is pretty opaque to the rest of the gRPC system, and it's controlled by the `SerializationTraits<T>` class. So you can take a gRPC stream reader and make it create any kind of type you want, even if the input data is a protocol buffer.

Some things that won't be addressed in this patch, as scope is too large:

* gRPC build toolchain issues (this is rather complex, I will create follow-up issues)
* Security / encryption, and authentication issues. I have only implemented an insecure server
* Integration with Travis CI
* Python bindings

API is preliminary and I expect to be the subject of iteration to make general and fast over the next several months.

Author: Wes McKinney <wesm+git@apache.org>

Closes #2547 from wesm/flight-cpp-prototype and squashes the following commits:

64bcdea43 <Wes McKinney> Initial Arrow Flight C++ implementation