Refactor of endpoint and connection management state logic

[neonphog]

• The main change of this PR is an inversion of the async driver for a tx5 endpoint.
  • Previously a central driver task would attempt to control all aspects, but the edge cases proved difficult to both reason about and test.
  • Now the main async driver is actually the send function. If we don't have a signal connection it will trigger creation of a cloneable future and await, following if there is no peer connection it will trigger creation of a cloneable peer connection future and await. The bulk of the complexity moves to ensuring these futures are cleaned up appropriately in error conditions.

[maackle]

woohoo!

[ThetaSinner]

Thanks for answering all my questions! Hopefully I contributed something as well as just learning :D PR looking good to me