Support Visual C++ and Windows (work in progress)

This PR tracks work-in-progress towards getting RocksDB compiling under Visual C++ on Windows. It's not ready to merge yet, but I'm opening it to get early feedback on the changes I'm making.

I expect the work to take place in at least five phases:
1. Remove incompatible C++ code, adding as few `#ifdef`s as possible.
2. Implement `port_win.cc` and `env_win.cc`.
3. Create Visual C++ project and Visual Studio solution.
4. Get third-party libraries (snappy, zlib, etc.) building and working.
5. Get the tests all compiling and running on Windows.

So far, step 1 is underway.