Cross platform file_io

This branch implements the changes need to the file_io component to support asynchronous I/O operations for the Windows platform.

In summary, the main component src/util/file_io.cpp is now a simple conditional structure to load with #ifdef a posix.cpp or windows.cpp implementation.

The signature of all the file_io methods remains the same, and only a few internal/utility methods were added to the Windows implementation.

async_file_handle and native_handle_t are aliases to avoid direct references to Posix types and to allow the usage of native Windows types.

The component src/util/file_io/random_access_handle_extended.hpp is key for the Windows implementation, as it adds the missing methods async_read_some and async_write_some to the random_access_handle class in order to keep unmodified the existent Ouinet code that is using file_io.