Support hot code reloading

[mkaput]

In order to let PullConsumer be full GenServer derivative, we must implement missing code_change/3 callback functionality. I'm not sure if this won't require more sophisticated support under the hood.

See:

• https://hexdocs.pm/elixir/1.13.3/GenServer.html#c:code_change/3
• https://hexdocs.pm/connection/Connection.html#c:code_change/3