Support "default" destination VPC, forbid other overlapping IPs

[qmonnet]

Follow-up to:

• https://github.com/githedgehog/internal/issues/287
• Peering API changes to include default gateway#275

Includes:

• Add flow-filter stage to assert packets belong to a valid peering connection #1158 (Quentin)
• Remove code for supporting overlapping exposed prefixes #1203 (Quentin)
• Adjust code for using default in the relevant components:
  • Support for default exposes #1198 (routing) (Fredi)
  • Add support for "default" destination to flow-filter stage #1205 (flow-filter, NAT) (Quentin)
• Config validation:
  • Add more validation for peerings with regards to "default", overlap #1207 (Quentin)