optimize disk cache io

[Kila2]

Utilize NIO instead of InputStream. For MacOS, NIO copy can employ fcopyfile for non-blocking IO copy to optimize IO performance.

[fmeum]

@tjgq Have there been any experiments at Google on replacing some of the custom filesystem functions with Java NIO? I recently used Files.move to replace the JavaIoFilesystem.renameTo implementation and wondered whether it would make sense to adopt it more generally.

[matthewjh]

Any update here? @Kila2 @fmeum @tjgq