Queue barcode event messages per notifier

[markusressel]

Is your feature request related to a problem? Please describe.
The MQTT / HTTP / Websocket connection can be lost temporarily due f.ex. network issues, which can result in the loss of barcode events on the client side.

Describe the solution you'd like
Implement an event queue, which stores events temporarily when the connection drops, to be able to resend them when the connection is reestablished.

For the MQTT and HTTP notifiers, a simple (infinite) retry loop should be sufficient.

For websocket clients, events should be queued per client.

Since the queue could become very large over time when the service uptime is really long, events in the queue of individual clients should be automatically dropped when they are older than a specified amount of time. This should default to 2 hours and be user configurable via the YAML config file.

[markusressel]

Implementing message queuing for temporarily disconnected websocket is difficult, since there is no unique identifier of websocket clients between connections. For this reason it will not be implemented for this issue, and is moved to #28 instead.