CJR limits block downloading blobs

[johnhferland]

What happened?

Outgoing requests to load blobs from data store block on acquiring permits. Permits don't increase with usage, even though there are no 429s or 503s returned from the data store.

Currently, CJR only releases limiters when the response has been fully read. This slows down the increase in permits for clients that load blobs.

What did you want to happen?

Have CJR release limiters once the client starts streaming the bytes of a response.