feat: SIGNAL-7190 Task.await_many for batch operations (#110)

## Related Ticket(s)
SIGNAL-7190
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- [x] Code conforms to the [Elixir
Styleguide](https://github.com/christopheradams/elixir_style_guide)

## Problem

Task.async operations to finish before finishing up the batch because
it’s “going too fast” and DB can’t keep up (afraid same thing might
happen in prod).

## Details

We’ll use Task.await_many as the back pressure, as that delays batching
function to finish, and increasing the time to finish is the back
pressure onto receiving more messages.

lib/kafee/consumer/broadway_adapter.ex

@@ -217,7 +217,8 @@ defmodule Kafee.Consumer.BroadwayAdapter do
     if batch_config[:async_run] do
       # No need for Task.Supervisor as it is not running under a GenServer,
       # and Kafee.Consumer.Adapter.push_message does already have error handling.
-      Enum.each(messages, &Task.async(fn -> do_consumer_work(&1, consumer, options) end))
+      tasks = Enum.map(messages, &Task.async(fn -> do_consumer_work(&1, consumer, options) end))
+      Task.await_many(tasks)
     else
       Enum.each(messages, fn message -> do_consumer_work(message, consumer, options) end)
     end