fix md5 hashing as MessageDigest is not threadsafe, now using ThreadLocal

src/main/java/redis/clients/util/Hashing.java

@@ -5,23 +5,22 @@ import java.security.NoSuchAlgorithmException;
 
 public interface Hashing {
     public static final Hashing MURMUR_HASH = new MurmurHash();
+    public ThreadLocal<MessageDigest> md5Holder = new ThreadLocal<MessageDigest>();
 
     public static final Hashing MD5 = new Hashing() {
-        private MessageDigest md5 = null; // avoid recurring construction
-
         public long hash(String key) {
             return hash(SafeEncoder.encode(key));
         }
 
         public long hash(byte[] key) {
-            if (md5 == null) {
-                try {
-                    md5 = MessageDigest.getInstance("MD5");
-                } catch (NoSuchAlgorithmException e) {
-                    throw new IllegalStateException(
-                            "++++ no md5 algorythm found");
+            try {
+                if (md5Holder.get() == null) {
+                    md5Holder.set(MessageDigest.getInstance("MD5"));
                 }
+            } catch (NoSuchAlgorithmException e) {
+                throw new IllegalStateException("++++ no md5 algorythm found");
             }
+            MessageDigest md5 = md5Holder.get();
 
             md5.reset();
             md5.update(key);