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ARROW-508: [C++] Add basic threadsafety to normal files and memory maps

This patch is stacked on ARROW-494, so will need to be rebased.

* Since the naive `ReadAt` implementation involves a Seek and a Read, this locks until the read is completed.
* Normal file reads block until completion
* File writes block until completion

This covers the threadsafety requirements for parquet-cpp at least. For on-disk files, the following methods are now threadsafe:

* `ArrowInputFile::Read` and `ArrowInputFile::ReadAt`
* `ArrowOutputStream::Write`

parquet-cpp calls `Seek` in a couple places:

https://github.com/apache/parquet-cpp/blob/master/src/parquet/file/reader-internal.cc#L257

Strictly speaking, if two threads are trying to read the same file from the same input source, this could have a race condition in esoteric circumstances. I'm going to report a bug to change these to `ReadAt` which can be more easily made threadsafe

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

Closes #300 from wesm/ARROW-508 and squashes the following commits:

e57156c [Wes McKinney] Make base ReadableFileInterface::ReadAt and some file functions threadsafe