thd->m_transaction_psi was not properly cleared for new connections

This happend when using XA transactions. I also added some extra asserts
to ensure that m_transactions are properly cleared.

Other things:
- Removed set_time() from THD::init_for_queries() as dispatch_command()
  is already doing that.
- Removed duplicate init_for_queries() from prepare_new_connection_state().
  The init_for_queries() functions should only be called once per
  connection.

sql/sql_connect.cc

@@ -1279,7 +1279,6 @@ void prepare_new_connection_state(THD* thd)
     }
 
     thd->proc_info=0;
-    thd->init_for_queries();
   }
 }