Asynchronous queries

Some external services will return results asynchronously. A good historical example of this is SDSS CasJobs; it's also supported (optionally?) by some(?) VO services, and it will certainly be required for many queries of the Rubin archive.

We need to support it within ESAP. This will include (presumably)

• Enabling the user to submit a query that doesn't return an immediate result;
• Presenting them with a list of queries that they have submitted which are in progress;
• Notifying them when results become available;
• Enabling them to retrieve and view the results at their leisure.