The current implementation uses separate goroutines to process each
message retrieved by `sqsClient.ReceiveMessageWithContext`, which is
executed when `concurrencyMode` is set to `parallel`.
The `consumeSubscription` function waits for all spawned goroutines to
complete before receiving the next batch. However, this design has a
critical flaw: it starves the consumer if new messages are published
during the processing of an existing batch. These newly published
messages won't be processed until all goroutines from the original batch
have finished their work, effectively blocking the consumption of the
new messages.

This PR changes that by removing the waiting mechanism.

Signed-off-by: Gustavo Chain <[email protected]>

The `ConcurrencyLimit` param controls the number of concurrent process
that handles messages.

Signed-off-by: Gustavo Chain <[email protected]>

Signed-off-by: Alessandro (Ale) Segala <[email protected]>