Converge: resource.needed_by is incorrect if resource type changed

[huangtianhua]

Enable convergence:
1. create a stack with resource A(name 'A'), and resource B(name 'B'), and resource B depends on A
2. update the stack: change resource A to another resource type, delete resource B
3. the stack update successful, but found the db data of resource A(new type).needed_by is incorrect, resource B still in it.

Though looks like anything is affected by this residue, will be good to correct.

[Chris Martin (cm876n)]

Removed assignee, no progress for 5 months