ARROW-1641: [C++] Hide std::mutex from public headers

[wesm]

This was one part of ARROW-1134 that we can push through. I had to do some refactoring since there was a mutex in one of the base file interfaces. It doesn't appear that this will impact parquet-cpp or other Arrow users

[wesm]

Hm, seems I have some more work to do here. I'm not sure about the best strategy for threadsafety by default in the RandomAccessFile::ReadAt functions. Before there was a default virtual implementation, but the mutex was visible in the header. We could add a PIMPL to RandomAccessFile but that might be overkill.

The default option is that I add basic implementations to the classes that are breaking the build currently

[wesm]

This should be ready to go once the tests pass

[wesm]

hm, there's a test failure, it looks like I borked something with threadsafety. taking another look

[wesm]

@kou it's possible that this patch introduces threadsafety issues in the GIO wrapper implementations if you use the RandomAccessFile::ReadAt functions at all, just as a heads up

[kou]

Thanks for the heads-up.
I'll add thread-safe ReadAt implementation to garrow::GIOInputStream later.

[wesm]

Waiting for https://ci.appveyor.com/project/wesm/arrow/build/1.0.1269 since I twiddled some symbol visibility stuff

[wesm]

+1