Adding support for using multiple RabbitMQ channels per connection

[bclougherty]

This ticket is an alternate attempt to address #57 by using multiple RabbitMQ channels per connection. Since each channel runs in its own goroutine, the processors are also running concurrently. This allows control over the degree of concurrency by specifying a fixed number of channels to use for any given instance, while maintaining a single TCP connection to the server.

The default behavior is to use a single channel, which will give exactly the same result as the current version, Adding --num-channels X at the command line will open multiple channels. As long as the GOMAXPROCS env variable is at least X, this will allow running X concurrent processors handing messages from the queue.

[corvus-ch]

Hi Brendan,

Thanks for your contribution. This one looks promising and I like the approach you took. I have made some comments and suggesting some improvements to make. Besides that, can you please have a look hat those integration tests(TEST_ARGS="-tags=integration" make test). They seem to be stuck somewhere.