s3:winbind: in idmap_tdb2_id_to_sid(), use transaction wrapped stores.

s3:winbind: in idmap_tdb2_id_to_sid(), use transaction wrapped stores.

When a mapping is not found, then the idmap script is called (if defined).
When this gives a reply for the desired id, this reply is stored in the db.

This patch wraps theses two store operations into a transaction by re-using
the idmap_tdb2_set_mapping_action() function previously defined
for idmap_tdb2_set_mapping().

Michael