Use amqp for broker/worker communication

[mweinelt]

Creates the WorkerRegistry object to
- track alive workers and their metrics
- submit client requests to workers

Creates the Worker object to
- hold a workers metric
- wrap queue interaction for the worker

Creates the Broker object to
- connect the HTTP API call to WorkerRegistry functions

Library wise these changes are included:
- Adds pika as the amqp connection library and bottle to replace the
  comparatively heavy flask and Werkzeug libraries.
  Bottle can be used with less global state and inside a class, which
  allows to track state within class members.

- Introduces structlog, for log messages that can also expose key/value
  pairs for structered logging.

[mweinelt]

Why AMQP?

• We can track delivery and acknowledgement of submitted jobs
• We can requeue them should they fail
• We can select which worker receives which jobs
• We can create a high-available setup with multiple wgkex-broker instances and multiple, clustered rabbitmq daemons (https://www.rabbitmq.com/ha.html)
  • gateways push their registration and metrics into the brokers queue, and the message gets replicated for all brokers

[awlx]

Merged the netlink branch. So we can rebase on that :). Maybe we can also reuse some of the ideas from mqtt and move on to amqp.

[awlx]

Replaced by #87