SPY-174: Make sure MultiOperationCallback is threadsafe

First, special thanks goes to @exortech for reporting and fixing the issue
here: #9

Motivation
----------
Certain operations like the multi get callback rely on the multi operation
callback which has a non-thread safe component to it.

Modification
------------
Make the callback atomic and thread safe.

Result
------
No race conditions with bulk get operation callbacks.

Change-Id: I511a8e5ec6e8fe50168337a4b9bbddf2360bd365
Reviewed-on: http://review.couchbase.org/38358
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>

Author: daschl

src/main/java/net/spy/memcached/ops/MultiOperationCallback.java

@@ -22,6 +22,8 @@
 
 package net.spy.memcached.ops;
 
+import java.util.concurrent.atomic.AtomicInteger;
+
 /**
  * An operation callback that will capture receivedStatus and complete
  * invocations and dispatch to a single callback.
@@ -34,7 +36,7 @@
 public abstract class MultiOperationCallback implements OperationCallback {
 
   private OperationStatus mostRecentStatus = null;
-  private int remaining = 0;
+  private final AtomicInteger remaining;
   protected final OperationCallback originalCallback;
 
   /**
@@ -50,11 +52,11 @@ public MultiOperationCallback(OperationCallback original, int todo) {
     }
 
     originalCallback = original;
-    remaining = todo;
+    remaining = new AtomicInteger(todo);
   }
 
   public void complete() {
-    if (--remaining == 0) {
+    if (remaining.decrementAndGet() == 0) {
       originalCallback.receivedStatus(mostRecentStatus);
       originalCallback.complete();
     }