Handle multiple sets of credentials with a single ApnsClient instance

[jchambers]

Implementing multiple sets of credentials within a single client (which will close #540) will be no small feat. For now, this is an integration branch that serves two purposes:

1. It provides a home for a big, wide-ranging development project without making undue noise in master (which I really need to get around to renaming to main one of these days)
2. It provides visibility into the effort

In time, this will become a more serious "for serious review" branch, but for now, it's just a draft and a bit of a holding area.