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.

dbsim/db_client_service.hpp

@@ -52,6 +52,10 @@ namespace db
         {
             return false;
         }
+        std::string xid() const override
+        {
+            return "";
+        }
         size_t bytes_generated() const override
         {
             return 0;