idmap_tdb2: fix a race condition in idmap_tdb2_allocate_id().

idmap_tdb2: fix a race condition in idmap_tdb2_allocate_id().

The race is a regression introduced by the change to dbwrap.
It might have led to two concurrent processes returning the same id.

This fix is achieved by changing dbwrap_change_uint32_atomic() to
match the original behaviour of tdb_change_uint32_atomic(), which
is the following: *oldval is used as initial value when
the value does not yet exist and that the old value should be
returned in *oldval.

dbwrap_change_uint32_atomic() is used (only) in idmap_tdb2.c,
to get new ids.

Michael