create uuid before sending PromoteMemory message
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When `Blob::promote` is promoting a `BlobImpl::Memory`, create uuid and send it with the `FileManagerThreadMsg::PromoteMemory` message rather than blocking until one is received.
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Fix FileAPI's refcount implementation
Revise several intricate parts of FileAPI's internal refcounting-related implementation.
Goal: Get it done right once and for all.
r? @Manishearth
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