Nanosecond timestamp support

[stevenxxiu]

Android kernels don't have NFS by default, so I'm trying to use this in Android.

I see however that only millisecond timestamp resolution is supported. NFS supports nanosecond timestamp per https://www.ietf.org/rfc/rfc3530.txt.

Can timestamps be changed to nanoseconds?

[kofemann]

The solution will be to update the org.dcache.nfs.vfs.Stat class to use Instant instead of long for various timestamps.