Release high prio client mutex before commit order wait

High priority transaction did not release mutex before going
into commit order wait. This allowed a deadlock if an applier
tried to access transaction state while the thread was in
commit order wait.

As a fix, unlock client mutex before calling commit_order_enter().

src/transaction.cpp

@@ -437,7 +437,9 @@ int wsrep::transaction::before_commit()
         {
             ret = 0;
         }
+        lock.unlock();
         ret = ret || provider().commit_order_enter(ws_handle_, ws_meta_);
+        lock.lock();
         if (ret)
         {
             state(lock, s_must_abort);