ChannelBroker: small improvements

[anaPerezGhiglia]

• Remove :low priority since it doesn't make sense anymore. The piece of code where it was being considered is now unreachable after #2300

    future_call?(not_before) ->
          State.reenqueue_contact(new_state, respondent.id, :low)
  
      defp future_call?(not_before) do
  
          DateTime.compare(not_before, Ask.SystemTime.time().now |> DateTime.add(60, :second)) == :gt
      end

  ☝️ The thing is that now (with the queue stored in the db) we only activate respondents which contact

  not_before <= not_before = SystemTime.time().now |> DateTime.add(60, :second)

  See ChannelBrokerState.activate_next_in_queue

  this closes #2289

• Invert calls to Survey and ChannelBrokerState when the ChannelBroker tries to activate the next queued contact and it's expired

  • See this slack thread for more context

Observations 👀

I had to do a workaround for inverting the calls. I had to keep the state in a variable and return that after the call to Survey.contact_attempt_expired. I'm not fan of this solution, but I think I prefer this to the previous version, but I read your opinions!