When gtid_mode=on and binary log is off, the server automatically
inserts the transaction's GTID into mysql.gtid_executed within
the transaction. But if the GTID has already been inserted into
the table by an explicit INSERT statement, the server crashes.

After the fix, the server does not crash in above case. Push a
warning to client if user is modifying the gtid_executed table
explicitly. Ignore the duplicate key error and log a warning
for it when writing transaction owned GTID into gtid_executed
table implicitly within the transaction, we did not push a
warning to client in the case, since it is slave SQL thread
or worker sometimes.

@ sql/binlog.cc
We invoke the warn_on_modify_gtid_table(...) in two places to
decrease the iterations to table list for improving performance.