Add message timestamp to the metadata dict.
Add message timestamp to the metadata dict.

tilequeue/worker.py
	-90b14d4 removed the expectation of `sqs_handle` containing a
	method that allows you to retrieve its "sent to queue"
	timestamp, to allow SQS to be stubbed out with something like
	`queue.file`, and added a timestamp instance variable to
	`CoordMessage` to allow it to be set on a per-queue basis.
	`SqsQueueWriter` is still getting the `SentTimestamp` via
	`sqs_handle`, rather than from the initially received
	`CoordMessage`'s `timestamp` attribute. Add it to the `metadata`
	dict inside `SqsQueueReader` instead, and use that inside
	`SqsQueueWriter`.

tilezen/tilequeue,mapzen/tilequeue