auto_increment on duplicate entry

The bug was that when INSERT_ID was used and the storage
engine was told to release any reserved but not used
auto_increment values, it set the highest auto_increment
value to INSERT_ID.

The fix was to check if the auto_increment value was forced
by user (INSERT_ID) or by slave-thread, i.e. not auto-
generated. So that it is only allowed to release generated
values.