jormungandr: Error handling in stat_manger for RabbitMQ

When rabbitMQ is re-initialized and jormungandr is running,
jormungandr cannot push stat masage to rabbitMQ even if
the connection exists.

If it happens the connection will have to be re-initialized.
we can use pool: http://ask.github.io/kombu/userguide/pools.html