BUG#50312 Warnings for unsafe sub-statement not returned to client

After BUG#36649, warnings for sub-statements are cleared when a 
new sub-statement is started. This is problematic since it suppresses
warnings for unsafe statements in some cases. It is important that we
always give a warning to the client, because the user needs to know
when there is a risk that the slave goes out of sync.
                        
We fixed the problem by generating warning messages for unsafe statements
while returning from a stored procedure, function, trigger or while
executing a top level statement.
                        
We also started checking unsafeness when both performance and log tables are
used. This is necessary after the performance schema which does a distinction
between performance and log tables.

mysql-test/suite/binlog/t/binlog_unsafe.test

@@ -468,6 +468,7 @@ BEGIN
 END|
 DELIMITER ;|
 --echo "One unsafe warning should be issued in the following statement"
+--error ER_STORED_FUNCTION_PREVENTS_SWITCH_SQL_LOG_BIN
 SELECT fun_check_log_bin();
 --echo "SQL_LOG_BIN should be ON still"
 SHOW VARIABLES LIKE "SQL_LOG_BIN";