Add signal handling

Problem/Motivation

This patch adds signal handling, to be able to gracefully stop the processing of the queue which is useful when running the queue processing in CLI, or you need to temporarily stop the queue running due to a code deployment (e.g. applicable when using Supervisor).

I decided to add the signal handling listeners specifically to advancedqueue:queue:process command and the reasoning behind that decision was that the Processor class might run in a non-CLI scenario, so it felt nice having it decoupled from that.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes