manage-consumers: Allow for subtyped consumers.

This is immediately useful for commentstree_q but may also help if we
move vote_q procs onto a single box.
Shard commentstree_q processing by Link ID.

This should better isolate large threads and increase batch
effectiveness.
amqp: Make it possible to add application headers to messages.

Useful for carrying extra information out-of-band from the content.