[rabbitmq] Bump default prefetch level to 64

This lets RabbitMQ keep pushing messages to us
without constantly having to wait for the next
ack to arrive.

See http://rubybunny.info/articles/queues.html#message_acknowledgements
for more information about manual acknowledgements
and QoS (prefetch).
typo
[rabbitmq] Finish RabbitMQ input implementation for CRuby

This will manually reconnect and initialize the plugin
on a network failure instead of using Bunny's automatic
recovery feature.

It is sufficient and keeps the recovery logic (e.g.
reconnection period) in the plugin.
[rabbitmq] Manually shut down consumer work pool when handling network failures