XC_Core version 3 update

Fixed XC_ChannelDownload bug that made it impossible to send uncompressed files.
16 byte aligned FVector4 class implemented for SSE usage.

Multithreading:
- Reimplemented the LZMA symbols for thread safety.
** Packages and mods using this feature will need a header upgrade and a rebuild.
- Added ThreadedLog and ThreadedLogFlush externals to allow logging from worker threads.
- Atomics.h inlines have proper MS compiler detection macros (work on both VC++6 and VS2015)