Make LocalFileLinux::xx_readv() honor the request for parallel delivery. But don't use the feature yet.

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.

Add ZgyUtils.alturl() for wrapping SDAPI.getSerializedContext(), as was done in the old ZGY. Implemented for C++, the Python Wrapper, and SdGlue.