Cleanup enhancement

[wenyingd]

This change is trying to use HAPI in cleanup logic, and remove unnecessary NSX API calls if a VPC is already deleted recursively. It also introduces the VPC deletion in parallel.

Test Done:

1. With a benchmark for 50 VPCs, 5 subnets/ 100 subnet ports / 5 subnet connection binding maps / 1 static routes / 1 vpc IP address allocation / 1 security policy configured per VPC, the new logic takes about 3m to complete the clean up job, while the original logic may take almost 30m to do the same thing.