Reorganize the Handler architecture to a pipeline architecture with plugins.
Now plugins can define additional handlers and the handlers can be organized into named pipelines. Modules are no longer the unit of a handler, now we use classes so we can assert interface conformance. The GLOBAL_PIPELINE is gone, replaced by the 'built-in' pipeline. The OWNER_PIPELINE is not yet replaced. I still need a few more tests of the basic pipeline architecture, although the individual handlers have pretty good coverage. Added the IHandler and IPipeline interfaces. Still broken, but not yet removed: Mailman/pipeline/moderate.py.
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