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/handler.cc

@@ -1818,6 +1818,10 @@ err:
     ha_rollback_trans(thd, all);
   else
   {
+    /*
+      We are not really doing a rollback here, but the code in trans_commit()
+      requres that m_transaction_psi is 0 when we return from this function.
+    */
     MYSQL_ROLLBACK_TRANSACTION(thd->m_transaction_psi);
     thd->m_transaction_psi= NULL;
     WSREP_DEBUG("rollback skipped %p %d",thd->rgi_slave,

sql/sql_class.cc

@@ -1396,9 +1396,11 @@ void THD::update_all_stats()
 
 void THD::init_for_queries()
 {
-  set_time(); 
-  ha_enable_transaction(this,TRUE);
+  DBUG_ASSERT(transaction->on);
+  DBUG_ASSERT(m_transaction_psi == NULL);
 
+  /* Set time for --init-file queries */
+  set_time();
   reset_root_defaults(mem_root, variables.query_alloc_block_size,
                       variables.query_prealloc_size);
   reset_root_defaults(&transaction->mem_root,
@@ -1550,6 +1552,8 @@ void THD::cleanup(void)
     trans_rollback(this);
 
   DBUG_ASSERT(open_tables == NULL);
+  DBUG_ASSERT(m_transaction_psi == NULL);
+
   /*
     If the thread was in the middle of an ongoing transaction (rolled
     back a few lines above) or under LOCK TABLES (unlocked the tables
@@ -1650,6 +1654,7 @@ void THD::reset_for_reuse()
   abort_on_warning= 0;
   free_connection_done= 0;
   m_command= COM_CONNECT;
+  transaction->on= 1;
 #if defined(ENABLED_PROFILING)
   profiling.reset();
 #endif

sql/sql_connect.cc

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

sql/xa.cc

@@ -843,6 +843,7 @@ bool trans_xa_detach(THD *thd)
 
   thd->transaction->all.ha_list= 0;
   thd->transaction->all.no_2pc= 0;
+  thd->m_transaction_psi= 0;
   return false;
 }