Use unbuffered channels for lambda and agent data

Currently, we are using a buffered channel by default for lambda and agent data (ref). This is not required anymore as we already have a buffer in [accumulator](https://github.com/elastic/apm-aws-lambda/blob/b420e160647066d253a8d2b1f20232ecf358a62e/accumulator/batch.go). The channel based buffer may cause additional issues with lifecycle management of the extension as it is possible that the buffer has a lot of data but the function doesn't have time to process that data and has been frozen due to inactivity.