4.x: Optimise msg_store recovery in case of large message file (backport #13250)

[mergify]

Proposed Changes

Since 4.0.0 (commit d45fbc3) the shared message store writes large messages into their own rdq files. This information can be utilised when scanning rdq files during recovery to avoid reading in the whole message body into memory unnecessarily.

This commit addresses the same issue that was addressed in 3.13.x by commit baeefbe (ie. appending a large binary together from 4MB chunks leaves a lot of garbage and memory fragmentation behind) but even more efficiently.

Large messages which were written before 4.0.0, which don't fully fill the rdq file, are still handled as before.

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This is an automatic backport of pull request #13250 done by [Mergify](https://mergify.com).