Support arbitrary Service Bus message sizes

[cgwrench]

Service Bus has a message size limit of 256KB for Standard tier Namespaces and 1MB for the Premium tier. The other queuing implementations don't have a similar limit. We should investigate ways to support larger message sizes when using the Service Bus implementation.

One option is to allow users to (optionally) register a plugin that will store message payloads in an Azure Storage Account, see, for example, ServiceBus.AttachmentPlugin.

Adding this support would negate the need, I believe, to make any changes for #24.