don't load plugins twice.

Not exactly sure where that feature crept in some time ago. It was suppressed by checking to see if a queue plugin was already registered, and then bailing out on subsequent register_hook runs. I noticed it in testing, b/c I didn't have a queue plugin loaded. This removes the duplicate calls to register_hook.

* adds caching of the AUTH methods. You can't add new plugins or register new
  hooks w/o restarting QP, so cache the list and avoid having to generate it on every connection.

* reduces several instances of duplicated code into new functions

* other PBP changes (early exits, less indention, fewer unnecessary parens,
  etc.)