Improving mmap_windows.go

[database64128]

There are a few issues with the current mmap implementation for Windows.

1. The file mapping handle can be closed right after calling MapViewOfFile. There's no need to keep it in a map.
2. On 32-bit systems, with large files, MapViewOfFile may return a partial mapping. The VirtualQuery should be called on the returned address to determine the actual size of the mapping, and return an error if the mapping is incomplete. See https://github.com/golang/go/blob/master/src/cmd/go/internal/mmap/mmap_windows.go.
3. The mapping is opened with read-only access. No need to call FlushViewOfFile at all.

I can open a PR to address these issues, if you are OK with it.

These changes would be essentially a rewrite of mmap_windows.go, so the special license can be removed as well.

[oschwald]

That all sounds great. I don't personally use Windows and would be thankful for any help improving its support.