bugfix: AMQP broker/backend with connection error handling

[yoonsio]

Fix AMQP broker/backend.

• 1 connection per broker, 1 channel per worker (https://stackoverflow.com/questions/10407760/is-there-a-performance-difference-between-pooling-connections-or-channels-in-rab)
• handle connection error using conn.NotifyClose(c)
• more options by avoiding hard coded names for queue and exchange

[m3nu]

Just started using this package in production and found this error after about 2 days. Is this related to this issue?

Exception (504) Reason: "channel/connection is not open" (AMQP)

Workaround for now: restart it every few hours.

I'm happy to debug or test any pre-release if it helps.