Avoid RPC roundtrips in list commands

Current implementation of various `list_XXX` commands require cross-node
roundtrip for every processed item - because `rabbitmqctl` target node
is responsible for gathering global list of all items of
interest (channels etc.) and then processing them one by one.

For example, listing 10000 channels evenly distributed across 3 nodes
where network has 1ms delay takes more than 10 seconds on my
machine. And with the proposed change listing will take almost the same
time as it'll take to gather this info locally. E.g. in the case above
listing now takes 0.7 second on the same machine with same 1ms delay.

It works by invoking emitting_map on every node, where it should send
info about only local items to aggregator, in an async fashion - as no
reply from aggregator is needed.

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