Introduce channel message receiver

[ono]

The current version spawns a middle process for every handler registration. This causes a race condition that the message is not coming to the client with a right sequence.

The PR introduces a GenServer which ensures only a single message receiver per channel and client process. This makes sure original sequence of all incoming messages from RabbitMQ via a channel.

[michaelklishin]

For those who run into errors that look like

GenServer.call(AMQP.Channel.ReceiverManager, {:register_handler, #PID<0.3861.0>, #PID<0.3849.0>, :consume, []}, 5000)

with this version: don't forget to Application.ensure_all_started(:amqp) before you attempt to connect. Code that does not consume (this is realistic in some test suites, for example) would never runs into this.

[ono]

@michaelklishin I am expecting that it's usually started by Elixir runtime automatically unless you have a specific setting in your mix.exs or you use old version of Elixir. Or is it when the app starts consumer before amqp app starts? Do you mind sharing more details?