arrow

ARROW-494: [C++] Extend lifetime of memory mapped data if any buffers reference it

If you read memory in an IPC scenario and then the `arrow::io::MemoryMappedFile` goes out of scope, before this patch the memory was being unmapped even if there are `arrow::Buffer` object referencing it.

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

Closes #298 from wesm/ARROW-494 and squashes the following commits:

60222e3 [Wes McKinney] clang-format
2960d17 [Wes McKinney] Add C++ unit test
d7d776a [Wes McKinney] Add Python unit test where memory mapped file is garbage collected
edf1295 [Wes McKinney] Reimplement memory map owner as Buffer subclass so that MemoryMappedFile can be safely destructed without invalidating Buffers referencing the mapped data