Single threaded operation

[tomsaleeba]

Steps:

• start the server
• post an image if none are already available
• request an image in original size
• request the same image again, note that it respond instantly
• trigger a long running request (like using ?thumbnail=True as per Improve thumbnail generation performance #2)
• request the first image again while the long running request is still going

Expected:
The server can respond to multiple requests at once. Maybe this is an operational issue and we can solve this with a load balancer as long as the LB knows a worker is busy and doesn't send more traffic to it until it's ready.

Actual:
The long running tasks blocks any other requests.