escaped field names

When in mixed or statement mode, the master logs LOAD DATA
queries by resorting to an Execute_load_query_log_event. This
event does not contain the original query, but a rewritten
version of it, which includes the table field names. However, the
rewrite does not escape the field names. If these names match a
reserved keyword, then the slave will stop with a syntax error
when executing the event.

We fix this by escaping the fields names as it happens already
for the table name.