sync files from smallest first to biggest last

[geoffm]

I'm not sure in what order the files are syunced actually, but it would be nice if it ordered them like dropbox, syncing files from smallest to biggest. This allow to complete much faster a bigger part of the multiple files to sync

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[guruz]

Related: #1633 #2128

Maybe we should do this as part of the activation of pipelining #4454

FYI @ogoffart