magma_dump - Print compressed/uncompressed value sizes

Print uncompressed/compressed value sizes (pre/post snappy compression) as part of --docs-by-key / --docs-by-seq.

Right now we only print them as part of --treedata option which isn't very helpful for the user as it prints all sstables for seqIndex. User has to understand seqIndex/levels, etc other concepts. Also it'll dump both fragmented/unfragmented docs.