✨ Prioritize reading blobs from the buffer pool before fetching from remote storage

[joocer]

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Some queries may be able to be completely responded to by buffered blobs.

Also reduces the likelihood of a read evicting blobs it wanted to read but evicting it and subsequently fetching - given we limit evictions to 32 per query, this isn't a huge problem, but could save close to half a second on GCS.