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ARROW-1214: [Python/C++] Add C++ functionality to more easily handle encapsulated IPC messages, Python bindings

This patch does a bunch of things:

* Decouples the RecordBatchStreamReader from the actual message iteration (which is handled by a new `arrow::ipc::MessageReader` interface
* Enables `arrow::ipc::Message` to hold all of the memory for a complete unit of data: metadata plus body
* Renames some IPC methods for better consistency (GetNextRecordBatch -> ReadNextRecordBatch)
* Adds function to serialize a complete encapsulated message to an `arrow::io::OutputStream*
* Add Python bindings for all of the above, introduce `pyarrow.Message`, `pyarrow.MessageReader`. Add `read_message` and `Message.serialize` functions for efficient memory round trips
* Add `pyarrow.read_record_batch` for reading a single record batch given a message and a known schema

Later we will want to add `pyarrow.read_schema`, but it seemed like a bit of work to make it work for dictionaries.

This implements the C++ analogue to ARROW-1047, which was for Java. Not sure why I didn't create a JIRA about this. cc @icexelloss

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #839 from wesm/ARROW-1214 and squashes the following commits:

07f1820a [Wes McKinney] Refactor to introduce MessageReader abstract type, use unique_ptr for messages instead of shared_ptr. First cut at Message, MessageReader Python API. Add read_message, C++/Python machinery for message roundtrips to Buffer, comparison. Add function to read RecordBatch from encapsulated message given schema.