Handle multiple inflight requests in BlobCache

[powerje]

Multiple requests for the same file will lead to failures in the form of:

Error Domain=NSCocoaErrorDomain Code=516 "“14c17b223a6b49dcb366699fe5516676.png” couldn’t be moved to “foo.bar.baz” because an item with the same name already exists."

Kumo probably should:

• Recognize this error and hide the failure, just returning the expected file that does exist
• Recognize multiple requests to the same resource and share the inflight request