Add read_only field attribute

Some fields of an object are created by the database automatically
like 'id' in service object, thus should not be changed in general.

Add a read_only field attribute to specify such situation, which
can only be set once and can't be changed any more.

Change-Id: I25ba81ed7ec0d16782a59851944a14792d4c84ce
Simplify filter_scheduler.populate_retry()

This is the exactly what dict.setdefault() is useful for.

Change-Id: I356af8f87b84fb23383c06080b8fd373054b5949
Merge "Add read_only field attribute"
Merge "Simplify filter_scheduler.populate_retry()"