Rename MessageQueueSettings to MessageQueueMetadata and deprecate
client config such as polling interval, which doesn't belong here.
Improved API for specifying a custom shard reading policy.
Update default retry delay to 10 seconds, 1 second was too aggressive
Minor cleanup
Expose the shard reader policy
Merge pull request #316 from elandau/queue

Rename MessageQueueSettings to MessageQueueMetadata and deprecate