s3:idmap_tdb2: add allocation of new mappings to idmap_tdb2_sids_to_unixids
This moves the new_mapping feature inside the tdb2 backend to make creations
of mappings atomic.
Note: The new internal function idmap_tdb2_get_new_id() that is used to allocate
a new unix id is prepared to function for multiple explicitly configured idmap
domains, but currently it does only work for the default domain. The extended
allocation support requires extension of the data base format to store multiple
counters (per domain). This will be added in a later step (TODO!).