sqs: Makes sure http/client reuse connections between requests

When using the SQS client with a large number of goroutines the local ports available are quickly exhausted because the connections are not being reused. The symptom is a large number of connections in TIME_WAIT state and the error "cannot assign requested address". This code makes sure that all content was read before closing the connection so the http/client can reuse it instead of creating a new one.