feat: [#55] Add Error handlers functionality (#56)

* Centralise defaults for Harmoniser::Options
* Bump bunny to its latest
* Use cancel_consumers_before_closing! for channels
* Remove warnings from Dockerfile
* Change severity of default channel errors log to WARN
* Introduce Harmoniser.configuration#on_error
* Introduce Harmoniser.configuration#handle_error
* Replace usages of logger with error severity with handle_error

feat: [#52] Selective retry for connection (#53)

* Remove SignalException from Connection using Thread::Queue to
  communicate main process with thread dedicated to launch harmoniser
* Refactor some bits for cli for better readability
* Retry connecting only when Harmoniser is the main process, i.e.
  Harmoniser.server? is true

feat: [#47] Listen to memory and disk alarms (#48)

* Remove usages of Harmoniser.connection
* Update README and LICENSE
* Introduce Connectable module for Publisher, Subscriber and Topology
* Refactor Connection:
** Move dynamic props for Connection as part of the Connection instance
** Use logger instance with dependency injection
* Use at_exit hook to maybe close Publisher connection when the code
  does not run as part of harmoniser process
* Attempt to close Publisher connection also as part of harmoniser
  process
* Attach callbacks for block/unblock at Connection

flags: [SKIP_MANUAL]

feat: [#45] Configure concurrency of Harmoniser process (#46)

* Introduce a concurrency example using Bunny
* Update Bunny to its latest, i.e. 2.23
* Add concurrency option for the CLI. It defaults to unbounded
  concurrency
* Introduce Launcher#stop to manage cancelling subscribers and closing
  connection instead of using at_exit hook.
* Introduce concurrency at Launcher#start
* Move recovery_attempt_started and recovery_completed callbacks to
  Connectable so that do not depend on Harmoniser but on connection
  method instead
* Create/Push tag anytime a merge to master happens.
* Make private HARMONISER constants defined at Publisher/Subscriber
* Close channel only when Topology#declare is invoked
* Introduce Harmoniser::Subscriber::Registry to accommodate the logic of
  the classes that are considered subscribers
* Enrich on_error callback for Channel with reply_code and reply_text
  when `channel.close` is received from RabbitMQ
* Kill process when ACK timeout is received
* Set default work pool shutdown timeout to 25 for bounded concurrency

feat: [#43] Improve O11y

fix: [#41] Exit ruby process only when harmoniser