Rewrite the logic used to handle Redis commands with clustering.

The reason for this change is to support the upcoming redis-cluster since
it has different behaviors compared to the client-side cluster implementation
provided by Predis. For example redis-cluster will not support key tags or
certain operations currently available with our client-side implementation.

lib/Predis/Command/TransactionExec.php

@@ -24,12 +24,4 @@ class TransactionExec extends AbstractCommand
     {
         return 'EXEC';
     }
-
-    /**
-     * {@inheritdoc}
-     */
-    protected function canBeHashed()
-    {
-        return false;
-    }
 }