[CODEC-268] Deprecate the NULL_HASHCODE.

Author: aherbert

src/main/java/org/apache/commons/codec/digest/MurmurHash3.java

@@ -53,7 +53,10 @@
 public final class MurmurHash3 {
     /**
      * A random number to use for a hash code.
+     *
+     * @deprecated This is not used internally and will be removed in a future release.
      */
+    @Deprecated
     public static final long NULL_HASHCODE = 2862933555777941757L;
 
     /**