Auto merge of #23045 - miller-time:promote-memory-blob, r=jdm

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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- [X] These changes fix #23032  (GitHub issue number if applicable)

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