If we're going to do the work to provide more optimized ways to zero files,
then I'd feel better with this being in a more common place, so that any other
zero-ers are likely to reuse it. It also makes testing easier/more obvious.
Also, because it's needed for correctness, fix the aligned_alloc for OSX, which
wasn't aligned, and use an actually aligned allocation function.
`deleteFile` existed in IAsyncFileSystem, so an incremental delete function
seems to belong more as a virtual method on IAsyncFileSystem than a static
method on IAsyncFile, and the naming should match.
As long as we're here, change IAsyncFile to declare a virtual destructor, so
that it has good and proper C++ behavior. I presume this is what was vaguely
intended by the default constructor definition that previously existed?
Evan Tschannen