ipcz: New tests and miscellaneous fixes

This introduces a few new integration tests to cover complex routing
behavior, including a large number of hops to exercise chaotic proxy
elimination and a large number of routes to exercise dynamic shared
memory allocation.

The new tests helped to uncover several bugs which are all fixed here:

- Transferred portals could drop parcels due to starting with an
  incorrect outgoing SequenceNumber (trivial 1-line fix in Router)
- Redundant introduction links were not being properly discarded
  (missing early return in Node::AcceptIntroduction)
- NodeLinks could have an inconsistent notion of whether they haven't
  been activated yet or have been activated and deactivated (split
  NodeLink::Create into CreateInactive and CreateActive, and
  ActivationState introduced instead of a bool `active_`)
- The sync reference driver could leak driver handles (fixed)
- The multiprocess reference driver could UAF its SocketTransport
  (changed SocketTransport to be RefCounted)

Bug: 1299283
Change-Id: I326526a260fb366c5a91fd74396a13acc7f4e772
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3787790
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/main@{#1030609}