Consumer should allow having several workers

[Keruspe]

For now, the consumer handles new deliveries one by one. It would be cool if we could say something like "handle 10 messages in parallel"

[Antti]

@Keruspe Did you try to run several consumers, where each is backed up by a single thread? Also it's a good idea to open a new channel for each thread interacting with a connection. So 1 channel <-> 1 thread.

[Keruspe]

I tried but for some reason only one of the consumers was actually consuming messages too.

I iterated on top on #39 though and got really awkward results, will update the issue

[Keruspe]

Fwiw, in my reproducer from #39 (https://github.com/Keruspe/rust-amqp-bugs) I also added an example of my attempt at having several consumers in several threads, which reproduces my "only one consumer actually consumes.

export AMQP_URL then

cargo run --bin publish-data
cargo run --bin multi-consumer