Change AuthenticationManager to class with lock

[FranzBusch]

Motivation

Currently, the JWT authentication is using two actors to isolate the state. This leads to thread hops for every APNS request which we can avoid with using a Lock instead.

Modification

This PR contains a couple of modifications to the JWT authentication handling:

• Refactor the two actors to a single class using a Lock
• Drop the dependency on Foundation from the JWT handling.
• Improve the logging
• Add tests to verify the behaviour of the new JWT generation.

Result

We now have a faster and better tested JWT handling.

[kylebrowning]

This is really smart and a great use of lock.

I especially like the simplification of the process and the breakout of variables.

Thank you for doing this! Tests should be running.