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.

include/wsrep/client_service.hpp

@@ -73,6 +73,9 @@ namespace wsrep
          */
         virtual void cleanup_transaction() = 0;
 
+        //
+        // XA
+        //
         /**
          * Return true if the current transactions is XA
          */
@@ -83,6 +86,11 @@ namespace wsrep
          */
         virtual bool is_xa_prepare() const = 0;
 
+        /**
+         * Return a string representing the xid of the transaction
+         */
+        virtual std::string xid() const = 0;
+
         //
         // Streaming
         //