Revert "Revert "switch to using march_hare instead of hot_bunnies""

This reverts commit 7f7017d238bb49610786b8ae3709fd581866407c.
use a fixed thread pool for March Hare

since messages are long lived, we need lots of threads for processing jobs.

“By default MarchHare will rely on the default RabbitMQ Java client’s executor service, which has a fixed size of 5 threads.”

from : http://blog.rubyrabbitmq.info/blog/page/3/