Reconnect the connection thread if broker shuts down

[t-persson]

Make the subscriber and publisher interfaces more clear and stable.
Reconnect if the rabbitmq broker shuts down.
Configurable parameters for how many threads the subscriber is
allowed to create.
Keep track of published messages in order to requeue them if broker
goes down.
Create a base RabbitMQ class for shared methods.

Applicable Issues

fixes: #15
fixes: #16

Description of the Change

Reconnect the subscribers and publishers if RabbitMQ should go down for some reason.

Alternate Designs

Redesigned the code to utilize the SelectConnection instead of BlockingConnection of pika.
SelectConnection means async calls instead of sync calls of the BlockingConnection.
The BlockingConnection makes more understandable code whereas the SelectConnection is more reliable (We don't have to force thread-safety into a non-thread-safe tool).

Benefits

Reconnection is always good. More reliable code.

Possible Drawbacks

Could make it harder to maintain if one has problems with the SelectConnection.

Sign-off

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Signed-off-by: Tobias Persson [email protected]

[magnusbaeck]

I think we should fix the default routing key as noted in my comment but that can be done in a follow-up PR if you like.