[Subnet Mgr] Subnet deletion to support parallel deletion of relevant resources and rollback

[xieus]

Issue Description In com.futurewei.alcor.subnet.controller.deleteSubnetState, multiple downstream resources related to a to-be-deleted subnet need to get deleted.

Those resources include:

• delete subnet routing rule in route manager
• delete gateway port in port manager
• delete subnet id in vpc
• delete ip range in Private IP Manager

In current implementation, resource deletion is sequential and unsupported of rollback if one deletion fails.

We want to support parallel deletion of relevant resources and rollback.