Support detach on transaction prepare

Split method `transaction::xa_detach()` in `before_xa_detach()` and
`after_xa_detach()`. This allows to handle bf abort or replay while
transaction is detaching and to distinguish if a transaction is bf
aborted before or after the DBMS has detached a transaction from its
storage engine.

include/wsrep/high_priority_service.hpp

@@ -63,6 +63,11 @@ namespace wsrep
          */
         virtual const wsrep::transaction& transaction() const = 0;
 
+        /**
+         * Return the associated client_state object
+         */
+        virtual wsrep::client_state& client_state() const = 0;
+
         /**
          * Adopt a transaction.
          */