Better validation for object removal (details inside)

[Frostman]

At least, not limited to:

• [x] Don't allow to delete VPC if VPC Attachments or VPC/Ext peerings exists
• [x] Don't allow to delete last management conn
• [x] Dont'allow to delete MCLAG Domain if there are MCLAG conns present
• [x] update docs as needed

[mrbojangles3]

@Frostman, do you think I can close this?