SYSDB: Check if group attributes differ before saving a group
Adds a new function sysdb_entry_attrs_diff() used in group saving code.
This function is used to check if the result of updating a group would
result in actually changing the sysdb entry -- often, we would try to
dump the same data to the cache during update. If that's the case, the
update code now only updates the timestamp cache, avoiding costly
Reviewed-by: Sumit Bose <email@example.com>