Add pool_idle_timeout options for s3 and sts clients

[mosyp]

A follow up of #451 (comment)

My local testing with these changes fixed the

 HttpDispatch(HttpDispatchError { message: "Error during dispatch: error writing a body to connection: Broken pipe (os error 32)" })

Since now, the client (rusoto/hyper) timeout (15 sec) is less than S3 server (20 sec), as such, the connection won't be closed by server.