cloudwatchlogs: Conserve unnecessary DescribeLogStreams calls
cloudwatchlogs: Conserve unnecessary DescribeLogStreams calls

Writes to new CloudWatch Logs streams do not require a sequence token
to be specified.  If we can discern that a stream was newly created,
forego the DescribeLogStreams API call so that we have a better chance
of remaining under the API limit.

segmentio/ecs-logs