Add async API

[anjackson]

The default synchronous API is brittle with long-running tasks, particularly when the service is under high load. Very long socket connect/read timeouts are hard to ensure as the kernel or network layers can override client settings.

We should add an async API endpoint that:

• Takes a POST request that defines the render parameters.
• Returns a render task resource (202 Accepted + Location ?)
• Other location returns 403 Forbidden or 504 Gateway Timeout until the item is ready.
• When done, returns a 200, returning the full GET URL.

Then need to mod the client to launch and poll until timeout rather than rely on socket timeouts. As well as timing out, the client would give up polling if the response was anything other than 403 Forbidden or 504 Gateway Timeout.