Support recovery of XA transactions

* Add method `restore_prepared_transaction` to `client_state` class
  which restores a transaction state from storage given its xid.
* Add method `commit_or_rollback_by_xid` to terminate prepared XA
  transactions by xid.
* Make sure that transactions in prepared state are not rolled back
  when their master fails/partitions away.

test/mock_client_state.hpp

@@ -141,6 +141,11 @@ namespace wsrep
             return is_xa_prepare_;
         }
 
+        std::string xid() const WSREP_OVERRIDE
+        {
+            return "";
+        }
+
         size_t bytes_generated() const WSREP_OVERRIDE
         {
             return bytes_generated_;