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.

test/mock_high_priority_service.hpp

@@ -85,7 +85,7 @@ namespace wsrep
         bool is_replaying() const WSREP_OVERRIDE { return replaying_; }
         void debug_crash(const char*) WSREP_OVERRIDE { /* Not in unit tests*/}
 
-        wsrep::client_state& client_state()
+        wsrep::client_state& client_state() const WSREP_OVERRIDE
         {
             return *client_state_;
         }