Three-party handoff/Level 3 RPC support

[Semisol]

This issue is for tracking 3PH implementation status. This supersedes #160 as there have been significant changes to the 3PH protocol since that implementation.

Progress tracking

• Rework of Network implementation, and updating schemas
  • Completed by 3PH part 1: Prepare for 3PH support #594
  • Fix failing tests and test network #596 fixes failing tests
• Implementation of initial 3PH support
  • No embargoes
    • As a temporary solution, we may refuse to proceed with 3PH if we have outstanding promises.
    • This option can be disabled for testing purposes in certain environments, but E-order can be violated
  • No call result forwarding: This only handles capability transfers.
  • Updates to the test network to make it support 3PH, and test caases
• 4+ party forwarding: If Network3PH.Forward allows it, Accept messages can be forwarded if there exists a longer chain of vats.
  • This should be done with the initial 3PH PR if possible.
• Implementation of embargo support for 3PH
• Three party call forwarding
  • This would be beneficial in less environments compared to 3PH.
  • New APIs will most likely be required and code rewrites.

Other considerations

• We should consider offering a basic VatNetwork implementation that is stable, using hostnames as vat identifiers, and possibly with TLS.
• Work to prevent possible DoS issues should be done, as some networks will be public
  • A continuation of Connection level memory limits #298 could be done.