should not use trampoline execution context when s3 access.

[ninneko]

https://github.com/ixias-net/ixias/blob/develop/framework/ixias-aws-s3/src/main/scala/ixias/aws/s3/backend/AmazonS3Backend.scala#L61

This may use trampoline execution context, but this includes blocking io.
So, we should use own execution context, or async s3 api.

see following links
https://www.playframework.com/documentation/2.6.x/ThreadPools
https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/basics-async.html