Share authentication tokens between connections within a client

[jchambers]

In an effort to fix #811, this introduces an AuthenticationTokenProvider that allows multiple connections within an ApnsClient instance to share a single authentication token, reducing the number of token rotations over time.

[jchambers]

One problem I spotted in review: TokenAuthenticationApnsClientHandler needs a way to report to the provider that it thinks the token has expired if we get a TooManyProviderTokenUpdates (edit: I meant ExpiredProviderToken; pardon the confusion) response from the server.

[jchambers]

TokenAuthenticationApnsClientHandler needs a way to report to the provider that it thinks the token has expired…

This is done.