publisher.rpc: queing policies

This change allow to configure the behavior of rpc publisher
when rabbitmq is down.

3 policies are available:
- default: wait until it comes back if rabbit_max_retries <= 0
           raise a exception if rabbit_max_retries > 0
- drop: don't publish the samples
- queue: create a local queue of 'max_queue_length' samples

Configuration of the policy is done via the publisher url
example: rpc://?policy=queue&max_queue_length=100

Change-Id: I41c9be9e1c760db1155711325434a1877e6dd7b2
Fixes: bug#1192918 bug#1189488