Unlock before releasing aborted transaction in provider.

Having aborted transaction holding a lock when releasing the
transaction in provider may cause a deadlock if:
- The transaction was BF aborted before it was known that the
  latest fragment was successfully replicated,
- Transaction was going to be released on provider side, but
  it waited for commit order,
- BF thread tried to grab lock for BF aborting, perhaps for
  second time.

As a fix, unlock the lock protecting victim transaction for
the duration of transaction release.

src/transaction.cpp

@@ -841,7 +841,9 @@ int wsrep::transaction::after_statement()
         {
             ret = release_commit_order(lock);
         }
+        lock.unlock();
         provider().release(ws_handle_);
+        lock.lock();
     }
 
     if (state() != s_executing && state() != s_prepared)