Make xx_eof() and xx_write() thread safe for local file access. Technically they don't need to be. But it adds robustness.

Yagni: Remove the Config class and derivatives. Meant to be an internal version of IOContext but was never fully used in the C++ version. Might potentially be brought back and used later.

Make the compression- and file factories thread safe. This is unlikely to be needed but it makes for a friendlier API.