JCBC-128: Add ZERO to the PersistTo ENUM.

This small changeset is the prerequisite for a larger change
in the JCBC client, so that it is possible to only wait for
proper replication and not for persistence too. See the
proper commit in couchbase-java-client tagged with JCBC-128
for more details and tests.

Change-Id: I3b1385a7a9553170a46d7ef72d6b537c0997b351
Reviewed-on: http://review.couchbase.org/21415
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>

src/main/java/net/spy/memcached/PersistTo.java

@@ -26,6 +26,11 @@
  * PersistTo codes for a Observe operation.
  */
 public enum PersistTo {
+
+  /**
+   * Don't wait for persistence on any nodes.
+   */
+  ZERO(0),
   /**
    * Persist to the Master. ONE implies MASTER.
    */