Deferreds in the plugin manager

[gdude2002]

We need to look at how to handle plugins that do stuff that require Deferreds to be used on startup or shutdown. Obviously, the way to do this is to return them to the plugin manager - but what do we do with them after that?

See 84fd5b5 for comments relating to where we may use Deferreds.

This is a high-priority blocker as it prevents us from reloading some plugins.

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[gdude2002]

I've largely added support for this; might want to go over it and check again.