Channel and connection refactor

[mattbennett]

A number of changes in this dev branch that ran for longer than intended:

• Drops support for py34 and py35
• Adds support for py38 and py39
• Refactors the client to be more DRYl; DependencyProvider now only differs in how it spawns threads
• ConnectionManager uses a content manager to clean up on exit:
  • Explicitly closes any open streams
  • Sets the stopped event; fixes a bug where clients could deadlock after connection failures
• Adds explicit handling for the http2 ConnectionTerminated event
• Timeout threads now abort when all relevant streams are closed; previously they ran forever (!)
• Stream.close() is now idempotent -- required because many different codepaths now call it, and they race.
• introduces the notion of channels:
  • ServerChannel: just refactors code out of the Entrypoint
  • ClientChannel:
    • automatically reconnects when a connection is terminated, allowing a client to retry
    • placeholder support for more advanced connection management e.g. load-balancing per https://github.com/grpc/grpc/blob/master/doc/load-balancing.md