Restore TLS authentication and move to a one-team-per-connection model

[jchambers]

We recently learned that the APNs server enforces a "one team per connection" rule, which has led us to make two big changes:

1. TLS authentication is now back to stay.
2. We've adjusted a lot of internals and public interfaces to better reflect the "one team per connection" rule.

There's a lot of stuff in here, but a lot of it is just un-deleting files and also importing work from the "move stuff into the handler instead of the client" effort (see #404). At the end of the day, I think this makes a lot of things a lot cleaner, but boy howdy is there a lot going on here. Feedback is greatly appreciated!

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TODO:

• Update README
• Update documentation (especially ApnsClient and ApnsClientBuilder)
• Add tests for ApnsSigningKey and ApnsVerificationKey