SPY-134: Make sure lastReadTimestamp is concurrency safe.

Motivation
----------
When the last read timestamp is read from each node, there has been some
suspicion that the (sometimes very long) reported times are flawed.

Tracking it down further, it was discovered that the timestamp was potentially
accessed by two or more threads, but not volatile.

Modifications
-------------
This changeset makes the timstamp volatile and also uses nanoTime instead
of currentTimeMillis for better accuracy.

Result
------
Since the timestamp is now thread safe, the actual value will now be shown
to the user.

Change-Id: I39e47408319f8adf3b2875a4a730232ecadeafe2
Reviewed-on: http://review.couchbase.org/37646
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>

Author: daschl

src/main/java/net/spy/memcached/protocol/TCPMemcachedNodeImpl.java

@@ -69,7 +69,7 @@ public abstract class TCPMemcachedNodeImpl extends SpyObject implements
   private CountDownLatch authLatch;
   private ArrayList<Operation> reconnectBlocked;
   private long defaultOpTimeout;
-  private long lastReadTimestamp = System.currentTimeMillis();
+  private volatile long lastReadTimestamp = System.nanoTime();
   private MemcachedConnection connection;
 
   // operation Future.get timeout counter
@@ -637,14 +637,14 @@ public final void setupForAuth() {
    * @return milliseconds since last read.
    */
   public long lastReadDelta() {
-    return System.currentTimeMillis() - lastReadTimestamp;
+    return TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - lastReadTimestamp);
   }
 
   /**
    * Mark this node as having just completed a read.
    */
   public void completedRead() {
-    lastReadTimestamp = System.currentTimeMillis();
+    lastReadTimestamp = System.nanoTime();
   }
 
   @Override