Change the server side NFS write gather delay from 10ms to 20ms. This

Change the server side NFS write gather delay from 10ms to 20ms.  This
avoids a boundary condition with the 100hz system clock and should slightly
improve write gather performance.

Use getmicrouptime() instead of getmicrotime() to record access cache
timestamps.  This prevents the client-side NFS attribute cache from
getting confused if the system time steps backwards.