Support terminate/2 callback in PullConsumer

[mkaput]

In order to let PullConsumer be full GenServer derivative, we must implement missing terminate/2 callback functionality. It should be called when the process dies, not necessarily when connection disconnects.

See:

• https://hexdocs.pm/elixir/1.13.3/GenServer.html#c:terminate/2
• https://hexdocs.pm/connection/Connection.html#c:terminate/2